configure, cmake, lib: more form api deprecation

Introduce a --enable-form-api configure option to control its inclusion
in builds. The condition name defined for it is CURL_DISABLE_FORM_API.

Form api code is dependent of MIME: configure and CMake handle this
dependency automatically: CMake by making it a dependent option
explicitly, configure by inheriting the MIME value by default and
rejecting explicit incompatible values.

"form-api" is now a new hidden test feature.

Update libcurl modules to respect this option and adjust tests
accordingly.

Closes #9621

tests/server/disabled.c

@@ -78,6 +78,9 @@ static const char *disabled[]={
 #endif
 #ifndef USE_XATTR
   "xattr",
+#endif
+#ifdef CURL_DISABLE_FORM_API
+  "form-api",
 #endif
   NULL
 };