Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Eliminate gdb_checkc member from GDB_DBGPORT(), it is never used.

Use polling behaviour for gdb_getc() where convenient, this edges us
closer to the console code.

/* -> /*- for copyright notices, minor format tweaks as necessary

Introduce the GDB debugger backend for the new KDB framework. The
backend improves over the old GDB support in the following ways:
o Unified implementation with minimal MD code.
o A simple interface for devices to register themselves as debug
ports, ala consoles.
o Compression by using run-length encoding.
o Implements GDB threading support.