Factor out in-stepping-range checks.

Factor out in-stepping-range checks.

This adds a function for doing within-thread's-stepping-range checks,
and converts a couple spots to use it.  Following patches will add
more uses.

gdb/
2013-05-23  Yao Qi  <yao@codesourcery.com>
	    Pedro Alves  <palves@redhat.com>

	* gdbthread.h (pc_in_thread_step_range): New declaration.
	* thread.c (pc_in_thread_step_range): New function.
	* infrun.c (handle_inferior_event): Use it.