Fix new inferior events output

Fix new inferior events output

Since f67c0c917150 ("Enable 'set print inferior-events' and improve
detach/fork/kill/exit messages"), when detaching a remote process, we
get, for detach against a remote target:

 (gdb) detach
 Detaching from program: ...., process 5388
 Ending remote debugging.
 [Inferior 1 (Thread 5388.5388) detached]
              ^^^^^^^^^^^^^^^^

That is incorrect, for it is printing a thread id as string while we
should be printing the process id instead.  I.e., either one of:

 [Inferior 1 (process 5388) detached]
 [Inferior 1 (Remote target) detached]

depending on remote stub support for the multi-process extensions.

Similarly, after killing a process, we're printing thread ids while we
should be printing process ids.  E.g., on native GNU/Linux:

 (gdb) k
 Kill the program being debugged? (y or n) y
 [Inferior 1 (Thread 0x7ffff7faa8c0 (LWP 30721)) has been killed]
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

while it should have been:

 Kill the program being debugged? (y or n) y
 [Inferior 1 (process 30721) has been killed]
              ^^^^^^^^^^^^^

There's a wording inconsistency between detach and kill:

 [Inferior 1 (process 30721) has been killed]
 [Inferior 1 (process 30721) detached]

Given we were already saying "detached" instead of "has been
detached", and we used to say just "exited", and given that the "has
been" doesn't really add any information, this commit changes the
message to just "killed":

 [Inferior 1 (process 30721) killed]

gdb/ChangeLog:
2018-04-25  Pedro Alves  <palves@redhat.com>

	* infcmd.c (kill_command): Print the pid as string, not the whole
	thread's ptid.  Add comment.  s/has been killed/killed/ in output
	message.
	* remote.c (remote_detach_1): Print the pid as string, not the
	whole thread's ptid.

gdb/testsuite/ChangeLog:
2018-04-25  Pedro Alves  <palves@redhat.com>

	* gdb.base/hook-stop.exp: Expect "killed" instead of "has been
	killed".
	* gdb.base/kill-after-signal.exp: Likewise.
	* gdb.threads/kill.exp: Likewise.