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

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

Modify kdb_trap() so that it re-calls the dbbe_trap function as long as
the debugger back-end has changed. This means that switching from ddb
to gdb no longer requires a "step" which can be dangerous on an
already-crashed kernel.

Also add a capability to get from the gdb back-end back to ddb, by
typing ^C in the console window.

While here, simplify kdb_sysctl_available() by using
sbuf_new_for_sysctl(), and use strlcpy() instead of strncpy() since the
strlcpy semantic is desired.

MFC after: 1 month

Add kdb_cpu_sync_icache(), intended to synchronize instruction
caches with data caches after writing to memory. This typically
is required to make breakpoints work on ia64 and powerpc. For
those architectures the function is implemented.

Wrap our drivers gdb_getc() function so that if it returns -1 we
try again. This way it matches the console behaviour and allows us
to share more 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.