gdb: make deprecated_cmd_warning i18n friendly

gdb: make deprecated_cmd_warning i18n friendly

Rewrite deprecated_cmd_warning to be i18n friendly.  While I'm going
through the function I also cleaned up some whitespace issues,
replaced uses of NULL with nullptr, and moved some comments to avoid
having to add { ... }.

Though the message being printed has a 'Warning: ' prefix I could have
changed from using printf_filtered to use warning, however, I haven't
done that in this commit as that would change what GDB outputs and I
wanted this commit NOT to change the output.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* cli/cli-decode.c (deprecated_cmd_warning): Use nullptr instead
	of NULL.  Don't print message piece by piece, but sentence at a
	time to allow internationalisation.  Some whitespace cleanup.