build: set -O3 and tune WinCE in CI, fix getpart, vtls_scache fallouts

- GHA/windows/WinCE:
  - set `-O3 -DNDEBUG` C flags manually for the CMake mingw32ce build.
    CMake doesn't recognize the platform and fails to add them. To match
    autotools (using `-O2`), and hit similar compiler warnings.
  - enable parallel builds for cmake.
  - tune parallelism for cmake using unity batches.
  - tune parallelism for autotools.

  Follow-up to 2a292c3984 #15975

- tests: fix potentially uninitialized value in `readline()` in
  `getpart.c`. Detected by gcc 4.4.0 `-O2` (Windows CE) jobs:
  ```
  tests/server/getpart.c: In function 'getpart':
  tests/server/getpart.c:298: error: 'datalen' may be used uninitialized in this function
  ```
  Ref: https://github.com/curl/curl/actions/runs/13522595237/job/37785147505?pr=16476#step:11:25
  Follow-up to 592880a3ca

- vtls_scache: rework returning pointer to avoid compiler warning seen
  with `-O3` gcc 4.4.0 builds (Windows CE/schannel):
  ```
  lib/vtls/schannel.c: In function 'schannel_connect_step1':
  lib/vtls/vtls_scache.c:975: error: dereferencing pointer 'old_cred.4474' does break strict-aliasing rules
  lib/vtls/vtls_scache.c:985: error: dereferencing pointer 'old_cred.4474' does break strict-aliasing rules
  lib/vtls/schannel.c:959: note: initialized from here
  ```
  Ref: https://github.com/curl/curl/actions/runs/13523868335/job/37789610845#step:9:25
  Follow-up to fa0ccd9f1f #15774

Closes #16476

lib/vtls/schannel.c

@@ -898,7 +898,6 @@ schannel_connect_step1(struct Curl_cfilter *cf, struct Curl_easy *data)
   unsigned char alpn_buffer[128];
 #endif
   SECURITY_STATUS sspi_status = SEC_E_OK;
-  struct Curl_schannel_cred *old_cred = NULL;
   CURLcode result;
 
   DEBUGASSERT(backend);
@@ -955,9 +954,10 @@ schannel_connect_step1(struct Curl_cfilter *cf, struct Curl_easy *data)
 
   /* check for an existing reusable credential handle */
   if(ssl_config->primary.cache_session) {
+    struct Curl_schannel_cred *old_cred;
     Curl_ssl_scache_lock(data);
-    if(Curl_ssl_scache_get_obj(cf, data, connssl->peer.scache_key,
-                               (void **)&old_cred)) {
+    old_cred = Curl_ssl_scache_get_obj(cf, data, connssl->peer.scache_key);
+    if(old_cred) {
       backend->cred = old_cred;
       DEBUGF(infof(data, "schannel: reusing existing credential handle"));
 

lib/vtls/sectransp.c

@@ -1333,8 +1333,9 @@ static CURLcode sectransp_connect_step1(struct Curl_cfilter *cf,
     size_t ssl_sessionid_len;
 
     Curl_ssl_scache_lock(data);
-    if(Curl_ssl_scache_get_obj(cf, data, connssl->peer.scache_key,
-                               (void **)&ssl_sessionid)) {
+    ssl_sessionid = Curl_ssl_scache_get_obj(cf, data,
+                                            connssl->peer.scache_key);
+    if(ssl_sessionid) {
       /* we got a session id, use it! */
       err = SSLSetPeerID(backend->ssl_ctx, ssl_sessionid,
                          strlen(ssl_sessionid));

lib/vtls/vtls_scache.c

@@ -962,31 +962,29 @@ out:
   return result;
 }
 
-bool Curl_ssl_scache_get_obj(struct Curl_cfilter *cf,
-                             struct Curl_easy *data,
-                             const char *ssl_peer_key,
-                             void **sobj)
+void *Curl_ssl_scache_get_obj(struct Curl_cfilter *cf,
+                              struct Curl_easy *data,
+                              const char *ssl_peer_key)
 {
   struct Curl_ssl_scache *scache = cf_ssl_scache_get(data);
   struct ssl_primary_config *conn_config = Curl_ssl_cf_get_primary_config(cf);
   struct Curl_ssl_scache_peer *peer = NULL;
   CURLcode result;
+  void *sobj;
 
-  *sobj = NULL;
   if(!scache)
-    return FALSE;
+    return NULL;
 
   result = cf_ssl_find_peer_by_key(data, scache, ssl_peer_key, conn_config,
                                    &peer);
   if(result)
-    return FALSE;
+    return NULL;
 
-  if(peer)
-    *sobj = peer->sobj;
+  sobj = peer ? peer->sobj : NULL;
 
   CURL_TRC_SSLS(data, "%s cached session for '%s'",
-                *sobj ? "Found" : "No", ssl_peer_key);
-  return !!*sobj;
+                sobj ? "Found" : "No", ssl_peer_key);
+  return sobj;
 }
 
 void Curl_ssl_scache_remove_all(struct Curl_cfilter *cf,

lib/vtls/vtls_scache.h

@@ -85,12 +85,11 @@ void Curl_ssl_scache_unlock(struct Curl_easy *data);
  * @param cf      the connection filter wanting to use it
  * @param data    the transfer involved
  * @param ssl_peer_key the key for lookup
- * @param sobj    on return, the object for the peer key or NULL
+ * @retval sobj   the object for the peer key or NULL
  */
-bool Curl_ssl_scache_get_obj(struct Curl_cfilter *cf,
-                             struct Curl_easy *data,
-                             const char *ssl_peer_key,
-                             void **sobj);
+void *Curl_ssl_scache_get_obj(struct Curl_cfilter *cf,
+                              struct Curl_easy *data,
+                              const char *ssl_peer_key);
 
 typedef void Curl_ssl_scache_obj_dtor(void *sobj);