Fix use-after-free in gdbserver

Fix use-after-free in gdbserver

-fsanitize=address pointed out a use-after-free in gdbserver.  In
particular, handle_detach could reference "process" after it was
deleted by detach_inferior.  Avoiding this also necessitated changing
target_ops::join to take a pid rather than a process_info*.

Tested by the buildbot using a few of the gdbserver builders.

gdb/gdbserver/ChangeLog
2018-11-29  Tom Tromey  <tom@tromey.com>

	* win32-low.c (win32_join): Take pid, not process.
	* target.h (struct target_ops) <join>: Change argument type.
	(join_inferior): Change argument name.
	* spu-low.c (spu_join): Take pid, not process.
	* server.c (handle_detach): Preserve pid before destroying
	process.
	* lynx-low.c (lynx_join): Take pid, not process.
	* linux-low.c (linux_join): Take pid, not process.