ddb: Enable 'thread <address>'

ddb: Enable 'thread <address>'

Currently, the 'thread' command (to switch the debugger to another thread)
only accepts decimal-encoded tids.  Use the same parsing logic as 'show
thread <arg>' to accept hex-encoded thread pointers in addition to
decimal-encoded tids.

Document the 'thread' command in ddb.4 and expand the 'show thread'
documentation to cover the tid usage.

Reported by:	bwidawsk
Reviewed by:	bwidawsk (earlier version), kib (earlier version), markj
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D16962