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/haproxy-clientip.md

@@ -0,0 +1,33 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: haproxy-clientip
+Arg: <IP address>
+Help: Sets client IP in HAProxy PROXY protocol v1 header
+Protocols: HTTP
+Added: 8.2.0
+Category: http proxy
+Multi: single
+See-also:
+  - proxy
+Example:
+  - --haproxy-clientip $IP
+---
+
+# `--haproxy-clientip`
+
+Sets a client IP in HAProxy PROXY protocol v1 header at the beginning of the
+connection.
+
+For valid requests, IPv4 addresses must be indicated as a series of exactly
+4 integers in the range [0..255] inclusive written in decimal representation
+separated by exactly one dot between each other. Heading zeroes are not
+permitted in front of numbers in order to avoid any possible confusion
+with octal numbers. IPv6 addresses must be indicated as series of 4 hexadecimal
+digits (upper or lower case) delimited by colons between each other, with the
+acceptance of one double colon sequence to replace the largest acceptable range
+of consecutive zeroes. The total number of decoded bits must exactly be 128.
+
+Otherwise, any string can be accepted for the client IP and get sent.
+
+It replaces --haproxy-protocol if used, it is not necessary to specify both flags.