Support the NO_COLOR environment variable

Support the NO_COLOR environment variable

I ran across this site:

    https://no-color.org/

... which lobbies for tools to recognize the NO_COLOR environment
variable and disable any terminal styling when it is seen.

This patch implements this for gdb.

Regression tested on x86-64 Fedora 38.

Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
Reviewed-by: Kevin Buettner <kevinb@redhat.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>