New function cli-utils.c:extract_arg_const

New function cli-utils.c:extract_arg_const

This function provides the exact same functionality as extract_arg,
except that it takes a "const char**" instead of a "char **".
It allows us also to re-implement extract_arg almost as a simple
wrapper around the new function.

gdb/ChangeLog:

        Pedro Alves  <palves@redhat.com>
        Joel Brobecker  <brobecker@adacore.com>

        * cli/cli-utils.h (extract_arg_const): Add declaration.
        * cli/cli-utils.c (extract_arg_const): New function.
        (extract_arg): Reimplement using extract_arg_const.