Rename current_inferior as current_thread in gdbserver

GDB has a function named "current_inferior" and gdbserver has a global
variable named "current_inferior", but the two are not equivalent;
indeed, gdbserver does not have any real equivalent of what GDB calls
an inferior.  What gdbserver's "current_inferior" is actually pointing
to is a structure describing the current thread.  This commit renames
current_inferior as current_thread in gdbserver to clarify this.  It
also renames the function "set_desired_inferior" to "set_desired_thread"
and renames various local variables from foo_inferior to foo_thread.

gdb/gdbserver/ChangeLog:

	* inferiors.h (current_inferior): Renamed as...
	(current_thread): New variable.  All uses updated.
	* linux-low.c (get_pc): Renamed saved_inferior as saved_thread.
	(maybe_move_out_of_jump_pad): Likewise.
	(cancel_breakpoint): Likewise.
	(linux_low_filter_event): Likewise.
	(wait_for_sigstop): Likewise.
	(linux_resume_one_lwp): Likewise.
	(need_step_over_p): Likewise.
	(start_step_over): Likewise.
	(linux_stabilize_threads): Renamed save_inferior as saved_thread.
	* linux-x86-low.c (x86_linux_update_xmltarget): Likewise.
	* proc-service.c (ps_lgetregs): Renamed reg_inferior as reg_thread
	and save_inferior as saved_thread.
	* regcache.c (get_thread_regcache): Renamed saved_inferior as
	saved_thread.
	(regcache_invalidate_thread): Likewise.
	* remote-utils.c (prepare_resume_reply): Likewise.
	* thread-db.c (thread_db_get_tls_address): Likewise.
	(disable_thread_event_reporting): Likewise.
	(remove_thread_event_breakpoints): Likewise.
	* tracepoint.c (gdb_agent_about_to_close): Renamed save_inferior
	as saved_thread.
	* target.h (set_desired_inferior): Renamed as...
	(set_desired_thread): New declaration.  All uses updated.
	* server.c (myresume): Updated comment to reference thread instead
	of inferior.
	(handle_serial_event): Likewise.
	(handle_target_event): Likewise.

gdb/gdbserver/linux-tile-low.c

@@ -158,7 +158,7 @@ tile_regs_info (void)
 static void
 tile_arch_setup (void)
 {
-  int pid = pid_of (current_inferior);
+  int pid = pid_of (current_thread);
   unsigned int machine;
   int is_elf64 = linux_pid_exe_is_elf_64_file (pid, &machine);