GDBserver: Pass process_info pointer to target_detach and target_join

GDBserver: Pass process_info pointer to target_detach and target_join

We start from a process_info pointer, pass down process->pid, and then
the target_detach and target_join implementations need to find the
process from the pid again.  Pass the process_info pointer down
directly instead.

gdb/gdbserver/ChangeLog:
2018-07-13  Pedro Alves  <palves@redhat.com>

	* linux-low.c (linux_detach, linux_join): Change parameter to
	process_info pointer instead of pid.  Adjust.
	* lynx-low.c (lynx_detach, lynx_join): Likewise.
	* nto-low.c (nto_detach): Likewise.
	* spu-low.c (spu_detach, spu_join): Likewise.
	* win32-low.c (win32_detach, win32_join): Likewise.
	* server.c (handle_detach, detach_or_kill_for_exit): Adjust.
	* target.h (struct target_ops) <detach, join>: Change parameter to
	process_info pointer instead of pid.  Adjust all implementations
	and callers.
	(detach_inferior, join_inferior): Rename 'pid' parameter to
	'proc'.