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/data/test1308

@@ -4,20 +4,21 @@
 unittest
 curl_formadd
 curl_formget
+FORM
 </keywords>
 </info>
 
 #
 # Client-side
 <client>
-<server>
-none
-</server>
 <features>
 unittest
 http
-Mime
+form-api
 </features>
+<server>
+none
+</server>
  <name>
 formpost unit tests
  </name>

tests/data/test516

@@ -3,6 +3,7 @@
 <keywords>
 HTTP
 HTTP POST
+FORM
 </keywords>
 </info>
 
@@ -22,7 +23,7 @@ OK
 # Client-side
 <client>
 <features>
-Mime
+form-api
 </features>
 <server>
 http

tests/data/test554

@@ -3,6 +3,7 @@
 <keywords>
 HTTP
 HTTP POST
+FORM
 </keywords>
 </info>
 
@@ -39,7 +40,7 @@ hello
 # Client-side
 <client>
 <features>
-Mime
+form-api
 </features>
 <server>
 http

tests/data/test587

@@ -3,6 +3,7 @@
 <keywords>
 HTTP
 HTTP POST
+FORM
 flaky
 </keywords>
 </info>
@@ -17,7 +18,7 @@ flaky
 # Client-side
 <client>
 <features>
-Mime
+form-api
 </features>
 <server>
 http

tests/data/test650

@@ -24,7 +24,7 @@ hello
 # Client-side
 <client>
 <features>
-Mime
+form-api
 </features>
 <server>
 http

tests/data/test651

@@ -24,7 +24,7 @@ hello
 # Client-side
 <client>
 <features>
-Mime
+form-api
 </features>
 <server>
 http

tests/data/test672

@@ -33,7 +33,7 @@ hello
 # Client-side
 <client>
 <features>
-Mime
+form-api
 </features>
 <server>
 http

tests/data/test673

@@ -33,7 +33,7 @@ hello
 # Client-side
 <client>
 <features>
-Mime
+form-api
 </features>
 <server>
 http