docs/cmdline: change to .md for cmdline docs

- switch all invidual files documenting command line options into .md,
   as the documentation is now markdown-looking.

 - made the parser treat 4-space indents as quotes

 - switch to building the curl.1 manpage using the "mainpage.idx" file,
   which lists the files to include to generate it, instead of using the
   previous page-footer/headers. Also, those files are now also .md
   ones, using the same format. I gave them underscore prefixes to make
   them sort separately:
   _NAME.md, _SYNOPSIS.md, _DESCRIPTION.md, _URL.md, _GLOBBING.md,
   _VARIABLES.md, _OUTPUT.md, _PROTOCOLS.md, _PROGRESS.md, _VERSION.md,
   _OPTIONS.md, _FILES.md, _ENVIRONMENT.md, _PROXYPREFIX.md,
   _EXITCODES.md, _BUGS.md, _AUTHORS.md, _WWW.md, _SEEALSO.md

 - updated test cases accordingly

Closes #12751

tests/data/test1026

@@ -28,7 +28,7 @@ curl --manual
 # Search for these two sentinel lines in the manual output; if they are found,
 # then chances are good the entire manual is there.
 <postcheck>
-perl -e 'open(IN,$ARGV[0]); my $lines=grep(/(curl\s*-\s*transfer\sa\s*URL)|(CONTRIBUTORS)/, <IN>); exit ($lines != 2); # Let this file pass an XML syntax check: </IN>' %LOGDIR/stdout%TESTNUMBER
+perl -e 'open(IN,$ARGV[0]); my $lines=grep(/(curl\s*-\s*transfer\sa\s*URL)|(AUTHORS)/, <IN>); exit ($lines != 2); # Let this file pass an XML syntax check: </IN>' %LOGDIR/stdout%TESTNUMBER
 </postcheck>
 </client>
 

tests/data/test1478

@@ -19,7 +19,7 @@ src/tool_listhelp.c is in sync with docs/cmdline-opts
 </name>
 
 <command type="perl">
-%SRCDIR/../docs/cmdline-opts/gen.pl listhelp %SRCDIR/../docs/cmdline-opts/*.d
+%SRCDIR/../docs/cmdline-opts/gen.pl listhelp %SRCDIR/../docs/cmdline-opts/*.md
 </command>
 </client>