Add support for threads in vax_bsd_nat_target

ptrace(2) PT_GETREGS/PT_SETREGS accepts thread id (LWP) as the 4th
argument for threads.

gdb/ChangeLog:

	* vax-bsd-nat.c (vaxbsd_supply_gregset): New variable lwp and pass
	it to the ptrace call.
	* vax-bsd-nat.c (vaxbsd_collect_gregset): Likewise.