gdb: make command classes be bitmaps

This commit makes it so GDB's command classes can be represented with a
single bit, allowing for a command to have multiple classes.  This is
primarily done as preparation for the next patch, but it can provide
value on its own as some commands could be described as belonging to
multiple classes, such as "record" being obscure and related to running
the inferior.

Approved-By: Tom Tromey <tom@tromey.com>