build: fix -Wunused-macros warnings, and related tidy-ups

- fix internal macro `AN_APPLE_OS` reused between sources without
  resetting it. It may potentially have left the system sha256
  function unused.
- fix to define `WOLFSSL_OPTIONS_IGNORE_SYS` so that it always applies
  to wolfSSL headers, also during feature detection.
- md4, md5, sha256: simplify fallback logic.
- delete 20+ unused macros.
- scope or move macros to avoid `-Wunused-macros` warnings.
- examples: delete unused code.

The warning detects macros defined but not used within the same C
source. It does not warn for macros defined in headers. It also works
with unity builds, but to a lesser extent.

Closes #20593

lib/vtls/x509asn1.c

@@ -50,6 +50,7 @@
 /* Largest supported ASN.1 structure. */
 #define CURL_ASN1_MAX                   ((size_t)0x40000)      /* 256K */
 
+#ifdef WANT_EXTRACT_CERTINFO
 /* ASN.1 classes. */
 /* #define CURL_ASN1_UNIVERSAL             0 */
 /* #define CURL_ASN1_APPLICATION           1 */
@@ -86,7 +87,6 @@
 /* #define CURL_ASN1_CHARACTER_STRING      29 */
 #define CURL_ASN1_BMP_STRING            30
 
-#ifdef WANT_EXTRACT_CERTINFO
 /* ASN.1 OID table entry. */
 struct Curl_OID {
   const char *numoid;  /* Dotted-numeric OID. */