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/interface.md

@@ -0,0 +1,25 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: interface
+Arg: <name>
+Help: Use network INTERFACE (or address)
+Category: connection
+Added: 7.3
+Multi: single
+See-also:
+  - dns-interface
+Example:
+  - --interface eth0 $URL
+---
+
+# `--interface`
+
+Perform an operation using a specified interface. You can enter interface
+name, IP address or host name. An example could look like:
+
+    curl --interface eth0:1 https://www.example.com/
+
+On Linux it can be used to specify a **VRF**, but the binary needs to either
+have **CAP_NET_RAW** or to be run as root. More information about Linux
+**VRF**: https://www.kernel.org/doc/Documentation/networking/vrf.txt