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

.github/scripts/cleancmd.pl

@@ -0,0 +1,52 @@
+#!/usr/bin/perl
+# Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+#
+# SPDX-License-Identifier: curl
+#
+# Input: a cmdline docs markdown, it gets modfied *in place*
+#
+# The main purpose is to strip off the leading meta-data part, but also to
+# clean up whatever else the spell checker might have a problem with that we
+# still deem is fine.
+
+my $header = 1;
+while(1) {
+    # set this if the markdown has no meta-data header to skip
+    if($ARGV[0] eq "--no-header") {
+        shift @ARGV;
+        $header = 0;
+    }
+    else {
+        last;
+    }
+}
+
+my $f = $ARGV[0];
+
+open(F, "<$f") or die;
+
+my $ignore = $header;
+my $sepcount = 0;
+my @out;
+while(<F>) {
+    if(/^---/ && $header) {
+        if(++$sepcount == 2) {
+            $ignore = 0;
+        }
+        next;
+    }
+    next if($ignore);
+
+    # strip out all long command line options
+    $_ =~ s/--[a-z0-9-]+//g;
+
+    # strip out https URLs, we don't want them spellchecked
+    $_ =~ s!https://[a-z0-9\#_/.-]+!!gi;
+
+    push @out, $_;
+}
+close(F);
+
+open(O, ">$f") or die;
+print O @out;
+close(O);

.github/scripts/spellcheck.words

@@ -50,6 +50,7 @@ backend
 backends
 backoff
 backticks
+balancers
 Baratov
 basename
 bashrc
@@ -445,6 +446,7 @@ Makefile
 makefiles
 malloc
 mallocs
+manpage
 maprintf
 Marek
 Mavrogiannopoulos