gdb(4): Support empty qSupported queries

Technically a client may send a qSupported query without specifying any
client features. We should respond with our supported list in that case
instead of bailing with error.

Reported by: rlibby
Reviewed by: emaste, rlibby, vangyzen
Sponsored by: Isilon
Differential Revision: https://reviews.freebsd.org/D26115