Introduce utility function find_inferior_ptid

This patch introduces find_inferior_ptid to replace the common idiom

  find_inferior_pid (ptid_get_pid (...));

It replaces all the instances of that idiom that I found with the new
function.

No significant changes before/after the patch in the regression suite
on amd64 linux.

gdb/ChangeLog:

	* inferior.c (find_inferior_ptid): New function.
	* inferior.h (find_inferior_ptid): New declaration.
	* ada-tasks.c (ada_get_task_number): Use find_inferior_ptid.
	* corelow.c (core_pid_to_str): Same.
	* darwin-nat.c (darwin_resume): Same.
	* infrun.c (fetch_inferior_event): Same.
	(get_inferior_stop_soon): Same.
	(handle_inferior_event): Same.
	(handle_signal_stop): Same.
	* linux-nat.c (resume_lwp): Same.
	(stop_wait_callback): Same.
	* mi/mi-interp.c (mi_new_thread): Same.
	(mi_thread_exit): Same.
	* proc-service.c (ps_pglobal_lookup): Same.
	* record-btrace.c (record_btrace_step_thread): Same.
	* remote-sim.c (gdbsim_close_inferior): Same.
	(gdbsim_resume): Same.
	(gdbsim_stop): Same.
	* sol2-tdep.c (sol2_core_pid_to_str): Same.
	* target.c (memory_xfer_partial_1): Same.
	(default_thread_address_space): Same.
	* thread.c (thread_change_ptid): Same.
	(switch_to_thread): Same.
	(do_restore_current_thread_cleanup): Same.