[patch] Add -q in help info and doc

[Eli Zaretskii]

> From: Mingjie Xing <mingjie.xing@gmail.com>
> Date: Fri, 6 Jun 2014 16:12:35 +0800
> 
> I notice that "-q" and "--silent" options are not mentioned in gdb's
> help information. This patch just adds them and updates the document.
> 
> Besides, most options in gdb document are using "-" prefix instead of "--".
> 
> ChangeLog
> 
>         * gdb.texinfo (Invoking GDB): Add -q and --quiet.
>         * main.c (print_gdb_help): Add -q and --silent.
> 
> Is it OK?

Fine with me, thanks.