Convert previous_inferior_ptid to strong reference to thread_info

Convert previous_inferior_ptid to strong reference to thread_info

I originally wrote this patch, because while working on some other
patch, I spotted a regression in the
gdb.multi/multi-target-no-resumed.exp.exp testcase.  Debugging the
issue, I realized that the problem was related to how I was using
previous_inferior_ptid to look up the thread the user had last
selected.  The problem is that previous_inferior_ptid alone doesn't
tell you which target that ptid is from, and I was just always using
the current target, which was incorrect.  Two different targets may
have threads with the same ptid.

I decided to fix this by replacing previous_inferior_ptid with a
strong reference to the thread, called previous_thread.

I have since found a new motivation for this change -- I would like to
tweak "info program" to not rely on get_last_target_status returning a
ptid that still exists in the thread list.  With both the follow_fork
changes later in this series, and the step-over-thread-exit changes,
that can happen, as we'll delete threads and not clear the last
waitstatus.

A new update_previous_thread function is added that can be used to
update previous_thread from inferior_ptid.  This must be called in
several places that really want to get rid of previous_thread thread,
and reset the thread id counter, otherwise we get regressions like
these:

  (gdb) info threads -gid
    Id   GId  Target Id                               Frame
 - * 1    1    Thread 2974541.2974541 "tids-gid-reset" main () at src/gdb/testsuite/gdb.multi/tids-gid-reset.c:21
 - (gdb) PASS: gdb.multi/tids-gid-reset.exp: single-inferior: after restart: info threads -gid
 + * 1    2    Thread 2958361.2958361 "tids-gid-reset" main () at src/gdb/testsuite/gdb.multi/tids-gid-reset.c:21
 + (gdb) FAIL: gdb.multi/tids-gid-reset.exp: single-inferior: after restart: info threads -gid

and:

  Core was generated by `build/gdb/testsuite/outputs/gdb.reverse/sigall-precsave/si'.
  Program terminated with signal SIGTRAP, Trace/breakpoint trap.
  #0  gen_ABRT () at src/gdb/testsuite/gdb.reverse/sigall-reverse.c:398
  398      kill (getpid (), SIGABRT);
 +[Current thread is 1 (LWP 2662066)]
  Restored records from core file build/gdb/testsuite/outputs/gdb.reverse/sigall-precsave/sigall.precsave.
  #0  gen_ABRT () at src/gdb/testsuite/gdb.reverse/sigall-reverse.c:398
  398      kill (getpid (), SIGABRT);

  continue
  Continuing.

 -Program received signal SIGABRT, Aborted.
 +Thread 1 received signal SIGABRT, Aborted.
  0x00007ffff7dfd55b in kill () at ../sysdeps/unix/syscall-template.S:78
  78     ../sysdeps/unix/syscall-template.S: No such file or directory.
 -(gdb) PASS: gdb.reverse/sigall-precsave.exp: sig-test-1: get signal ABRT
 +(gdb) FAIL: gdb.reverse/sigall-precsave.exp: sig-test-1: get signal ABRT

I.e., GDB was failing to restart the thread counter back to 1, because
the previous_thread thread was being help due to the strong reference.

Tested on GNU/Linux native, gdbserver and gdbserver + "maint set
target-non-stop on".

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* infcmd.c (kill_command, detach_command, disconnect_command):
	Call update_previous_thread.
	* infrun.c (previous_inferior_ptid): Delete.
	(previous_thread): New.
	(update_previous_thread): New.
	(proceed, init_wait_for_inferior): Call update_previous_thread.
	(normal_stop): Adjust to compare previous_thread and
	inferior_thread.  Call update_previous_thread.
	* infrun.h (update_previous_thread): Declare.
	* target.c (target_pre_inferior, target_preopen): Call
	update_previous_thread.

Change-Id: I42779a1ee51a996fa1e8f6e1525c6605dbfd42c7