cmdline/docs: add a required 'multi' keyword for each option

The keyword specifies how option works when specified multiple times:

 - single: the last provided value replaces the earlier ones
 - append: it supports being provided multiple times
 - boolean: on/off values
 - mutex: flag-like option that disable anoter flag

The 'gen.pl' script then outputs the proper and unified language for
each option's multi-use behavior in the generated man page.

The multi: header is requires in each .d file and will cause build error
if missing or set to an unknown value.

Closes #9759

docs/cmdline-opts/basic.d

@@ -7,6 +7,7 @@ Protocols: HTTP
 Category: auth
 Example: -u name:password --basic $URL
 Added: 7.10.6
+Multi: mutex
 ---
 Tells curl to use HTTP Basic authentication with the remote host. This is the
 default and this option is usually pointless, unless you use it to override a