share: concurrency handling, easy updates

Replace the `volatile int dirty` with a reference counter
protected by a mutex when available.

Solve the problem of when to call application's lock function
by adding a volatile flag that indicates a share has been added
to easy handles in its lifetime. That flag ever goes from
FALSE to TRUE, so volatile might work (in the absence of a mutex).

(The problem is that the lock/unlock functions need 2-3
`curl_share_setopt()` invocations to become usable and there
is no way of telling if the third will ever happen. Calling
the lock function before the 3rd setopt may crash the
application.)

When removing a share from an easy handle (or replacing it with
another share), detach the easy connection on a share with a
connection pool.

When cleaning up a share, allow this even if it is still used in
easy handles. It will be destroyed when the reference count
drops to 0.

Closes #20870

lib/hostip4.c

@@ -85,7 +85,8 @@ struct Curl_addrinfo *Curl_sync_getaddrinfo(struct Curl_easy *data,
 #endif /* CURLRES_SYNCH */
 #endif /* CURLRES_IPV4 */
 
-#if defined(CURLRES_IPV4) && !defined(CURLRES_ARES) && !defined(CURLRES_AMIGA)
+#if defined(CURLRES_IPV4) && !defined(USE_RESOLV_ARES) && \
+    !defined(CURLRES_AMIGA)
 
 /*
  * Curl_ipv4_resolve_r() - ipv4 thread-safe resolver function.
@@ -277,4 +278,4 @@ struct Curl_addrinfo *Curl_ipv4_resolve_r(const char *hostname,
 
   return ai;
 }
-#endif /* CURLRES_IPV4 && !CURLRES_ARES && !CURLRES_AMIGA */
+#endif /* CURLRES_IPV4 && !USE_RESOLV_ARES && !CURLRES_AMIGA */