docs/libcurl: generate PROTOCOLS from meta-data

Remove the PROTOCOLS section from the source files completely and
instead generate them based on the header data in the curldown files.

It also generates TLS backend information for options marked for TLS as
protocol.

Closes #13175

docs/libcurl/opts/CURLOPT_NETRC_FILE.md

@@ -9,7 +9,7 @@ See-also:
   - CURLOPT_PASSWORD (3)
   - CURLOPT_USERNAME (3)
 Protocol:
-  - *
+  - All
 ---
 
 # NAME
@@ -38,10 +38,6 @@ option.
 
 NULL
 
-# PROTOCOLS
-
-All
-
 # EXAMPLE
 
 ~~~c