tap driver: handle signals received by the tests being run

tap driver: handle signals received by the tests being run

* lib/tap-driver (get_test_exit_message): Also deal with signals,
by using the `wait' method of the TAP::Parser object instead of
the `exit' method.  This required the use of the standard perl
module `POSIX'.
* doc/automake.texi (Use TAP with the Automake test harness):
Document that `--ignore-exit' has effect also on terminating
signals.  Add a "synchronizing" comment that references the tests
'tap-exit.test' and 'tap-signal.test'.
* tests/tap-signal.test: Extend and adjust.