x86: implement kdb watchpoint functions

Add wrappers around the dbreg interface that can be consumed by MI
kernel debugger code. The dbreg functions themselves are updated to
return error codes, not just -1. dbreg_set_watchpoint() is extended to
accept access bits as an argument.

Reviewed by: jhb, kib, markj
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D29155