Bug#253029: xmessage: could check if -default args are valid

[Dan Jacobson <jidanni@jidanni.org>]

Package: xbase-clients
Version: 4.3.0-7
Severity: minor
File: /usr/X11R6/bin/xmessage

Could check if -default args are one of the valid buttons, e.g., raise
an error for:
$ xmessage -buttons a,b,c -default ccc hi

BTW, could implement
$ xmessage --help
instead of engaging gears...