timeout: add --foreground to support interactive commands

timeout: add --foreground to support interactive commands

Or more accurately, commands not started from the shell prompt,
that are interactive, or need to receive Ctrl-C etc. from the terminal.

* doc/coreutils.texi (timeout invocation): Document --foreground.
* src/timeout.c (main): Set the foreground flag and don't create
a separate group.
(cleanup): Only send a signal directly to the monitored command
when the foreground flag is set.
(usage): Describe --foreground.
* tests/misc/timeout-group: Add a new test.
* tests/Makefile.am: Reference new test.
NEWS: Mention the new option.

Reported by Shay Shimony
Analysis by Alan Curry
Fix suggested by Paul Eggert