Avoid gdb.base/fork-running-state.exp lingering processes

Avoid gdb.base/fork-running-state.exp lingering processes

Currently, the gdb.base/fork-running-state.exp testcase leaves a few
processes lingering until a 3 minutes alarm kills them:

 pedro    28308     1  0 13:55 ?        00:00:00 /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/fork-running-state/fork-running-state
 pedro    28340     1  0 13:55 ?        00:00:00 /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/fork-running-state/fork-running-state
 pedro    28372     1  0 13:55 ?        00:00:00 /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/fork-running-state/fork-running-state
 pedro    28400     1  0 13:55 ?        00:00:00 /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/fork-running-state/fork-running-state
 pedro    28431     1  0 13:55 ?        00:00:00 /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/fork-running-state/fork-running-state
 pedro    28463     1  0 13:55 ?        00:00:00 /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/fork-running-state/fork-running-state

Those processes used to kill themselves, but that was changed by
commit f50d8a2eaea0 ("Fix gdb.base/fork-running-state.exp race").

This commit restores the self-killing, but only in the cases gdb won't
try killing the processes, thus avoiding the old race.

(The restored code in fork_parent isn't exactly the same as it was.
In this version, we're exiting immediately when 'wait' returns
success, while in the old version we'd loop again and end up in the
perror call.  The output from that perror call is not expected by the
"kill inferior" tests, and would result in a test FAIL.)

gdb/testsuite/ChangeLog:
2018-06-14  Pedro Alves  <palves@redhat.com>

	* gdb.base/fork-running-state.c: Include <errno.h>.
	(exit_if_relative_exits): New.
	(fork_child): If 'exit_if_relative_exits' is true, exit if the parent
	exits.
	(fork_parent): If 'exit_if_relative_exits' is true, exit if the
	child exits.