[Toybox] [PATCH] timeout: --foreground, --preserve-status, and --signal.

[enh]

--signal is simply a synonym for the exiting -s.

--foreground disables functionality we didn't yet have: putting the
child into a new process group. I've added the functionality and the
flag to disable it.

--preserve-status also makes it clear that our exit statuses didn't match
the coreutils version. In addition to callers that use --preserve-status
to get away from this madness, I also have callers that check for
specific exit values. This patch implements --preserve-status but also
fixes all the other exit statuses.

(The "125" exit value is broken for toybox in the same way that
`toybox grep --whoops ; echo $?` is. To fix this, we'd need some way to
signal that command-line parsing failures should exit with a different
value than the usual 1 --- 2 for grep, 125 for timeout. I've done as much
as grep manages, and left a TODO.)

Also add timeout tests. I couldn't think of an easy test for
--foreground, so I tested that manually with strace.

Also add some newlines to the `toybox --help` output to make it easier
to find the different sections, and expand the section on durations to
call out that fractions are supported as a matter of policy.

As long as timeout and sleep have text describing the duration syntax,
make them the same. (Personally I'd remove both in favor of the `toybox
--help` output, but as long as they're duplicated, keep them consistent.)

Also remove the SLEEP_FLOAT variant --- xparsetime means that sleep no
longer requires floating point to support sub-second resolution.
---
 Config.in            | 11 +++++++----
 tests/timeout.test   | 19 +++++++++++++++++++
 toys/other/timeout.c | 38 ++++++++++++++++++++++++++++++--------
 toys/posix/sleep.c   | 14 ++++----------
 4 files changed, 60 insertions(+), 22 deletions(-)
 create mode 100644 tests/timeout.test
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