timeout: add the --kill-after option

timeout: add the --kill-after option

Based on a report from Kim Hansen who wanted to
send a KILL signal to the monitored command
when `timeout` itself received a termination signal.
Rather than changing such a signal into a KILL,
we provide the more general mechanism of sending
the KILL after the specified grace period.

* src/timeout.c (cleanup): If a non zero kill delay
is specified, (re)set the alarm to that delay, after
which a KILL signal will be sent to the process group.
(usage): Mention the new option.  Separate the description
of DURATION since it's now specified in 2 places.
Clarify that the duration is an integer.
(parse_duration): A new function refactored from main(),
since this logic is now called for two parameters.
(main): Parse the -k option.
* doc/coreutils.texi (timeout invocation): Describe the
new --kill-after option and use @display rather than
@table to show the duration suffixes.  Clarify that
a duration of 0 disables the associated timeout.
* tests/misc/timeout-parameters: Check invalid --kill-after.
* tests/misc/timeout: Check a valid --kill-after works.
* NEWS: Mention the new feature.