Use kinfo_getfile to implement fdwalk on FreeBSD.

Use kinfo_getfile to implement fdwalk on FreeBSD.

kinfo_getfile() requires a couple of system calls to fetch the list of
open file descriptors.  This can be much cheaper than invoking fstat
on all of the values from 0 to the open file resource limit maximum.

gdb/ChangeLog:

	* common/filestuff.c [HAVE_KINFO_GETFILE]: Include headers.
	(fdwalk) [HAVE_KINFO_GETFILE]: Use kinfo_getfile.