According to the coreutils regression testsuite for timeout(1)
It is expect to exit with a status being:
125 in case an invalid duration or signal is passed in arguments
126 in case an invalid command is passed in arguments
127 in case the command passed in arguments does not exists.
While here document this behaviour in the man page
Document the exit values and the duration format
Improve wording
Pet mandoc -Tlint
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Phabric: https://phabric.freebsd.org/D432
Reviewed by: wblock
it fully passes the GNU timeout regression tests, it is written in a mostly
portable way (only signal parsing is relying on non portable structures)
Phabric: D377
