Hello,

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?

Best regards,
Mingjie