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

docs/cmdline-opts/form-string.md

@@ -0,0 +1,23 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: form-string
+Help: Specify multipart MIME data
+Protocols: HTTP SMTP IMAP
+Arg: <name=string>
+Category: http upload
+Added: 7.13.2
+Multi: append
+See-also:
+  - form
+Example:
+  - --form-string "data" $URL
+---
+
+# `--form-string`
+
+Similar to --form except that the value string for the named parameter is used
+literally. Leading '@' and '<' characters, and the ';type=' string in
+the value have no special meaning. Use this in preference to --form if
+there is any possibility that the string value may accidentally trigger the
+'@' or '<' features of --form.