gdb/

gdb/
	* gdbthread.h (currently_stepping): New declaration.
	* infrun.c (currently_stepping): Remove the forward declaration.
	(currently_stepping): Make it global.
	* linux-nat.c (resume_callback) <lp->stopped && lp->status == 0>: New
	variables tp and step, initialized them.  Pass STEP to to_resume.
	Print also possibly "PTRACE_SINGLESTEP" if STEP.  Initialize LP->STEP.
	* remote.c (currently_stepping_callback): New.
	(remote_vcont_resume)
	<ptid_equal (ptid, minus_one_ptid) || ptid_is_pid (ptid)>:
	New variable tp.  Call currently_stepping_callback and step such
	thread.

gdb/testsuite/
	* gdb.threads/sigstep-threads.exp: New file.
	* gdb.threads/sigstep-threads.c: New file.