bsd-uthread: Use ptid from regcache instead of inferior_ptid

bsd-uthread: Use ptid from regcache instead of inferior_ptid

This is one of the rare to_fetch/store_registers implementations that will
still rely (for now) on inferior_ptid (because of the memory read/write
operations).  We therefore have to add a save/restore of inferior_ptid.  We'll
be able to remove it when we make the memory operations accept the ptid as a
parameter.

gdb/ChangeLog:

	* bsd-uthread.c (bsd_uthread_fetch_registers,
	bsd_uthread_store_registers): Use ptid from regcache, set and
	restore inferior_ptid.