Add fbsd_nat_add_target.

Add fbsd_nat_add_target.

Add a wrapper for add_target in fbsd-nat.c to override target operations
common to all native FreeBSD targets.

gdb/ChangeLog:

	* fbsd-nat.c (fbsd_pid_to_exec_file): Mark static.
	(fbsd_find_memory_regions): Mark static.
	(fbsd_nat_add_target): New function.
	* fbsd-nat.h: Export fbsd_nat_add_target and remove prototypes for
	fbsd_pid_to_exec_file and fbsd_find_memory_regions.
	* amd64fbsd-nat.c (_initialize_amd64fbsd_nat): Use fbsd_nat_add_target.
	* i386fbsd-nat.c (_initialize_i386fbsd_nat): Likewise.
	* ppcfbsd-nat.c (_initialize_ppcfbsd_nat): Likewise.
	* sparc64fbsd-nat.c (_initialize_sparc64fbsd_nat): Likewise.