Switch local native code to gdb/nat shared functions

Switch local native code to gdb/nat shared functions

No functional change as the same functionality inlined in nbsd-nat.c
is offered in gdb/nat/netbsd-nat.c.

gdb/ChangeLog:

	* nbsd-nat.c: Include "nat/netbsd-nat.h".
	* (nbsd_nat_target::pid_to_exec_file)
	(nbsd_nat_target::thread_alive, nbsd_nat_target::thread_name)
	(nbsd_nat_target::post_startup_inferior)
	(nbsd_nat_target::post_attach, nbsd_nat_target::xfer_partial)
	(nbsd_add_threads): Switch local code to common gdb/nat functions.
	* (nbsd_pid_to_cmdline): Call sysctl from the global namespace.
	* (nbsd_thread_lister): Remove.