Add a new debug knob for the FreeBSD native target.

For now this just logs information about the state of the current LWP
for each STOPPED event in fbsd_wait().

gdb/ChangeLog:

	* NEWS (Changes since GDB 8.1): Add "set/show debug fbsd-nat".
	* fbsd-nat.c (debug_fbsd_nat): New variable.
	(show_fbsd_nat_debug): New function.
	(fbsd_wait): Log LWP info if "debug_fbsd_nat" is enabled.
	(_initialize_fbsd_nat): Add "fbsd-nat" debug boolean command.

gdb/doc/ChangeLog:

	* gdb.texinfo (Debugging Output): Document "set/show debug
	fbsd-nat".