kill: fix 'kill -0'

[ivy]

Summary

The switch to str2sig() added handling for 'kill -s 0', but not
'kill -0', which is required by POSIX (as well as /etc/rc.subr).

This caused rc.d scripts with the 'stop' or 'restart' command to
not correctly wait for the service to stop.

Fixes: 36679f7d7b56 ("kill: Use POSIX str2sig()")

[kib]

bin/kill/kill.c

102

Does e.g. "0000" qualify as zero?

[ivy]

bin/kill/kill.c

102

according to POSIX (https://pubs.opengroup.org/onlinepubs/9799919799/utilities/kill.html), -s should only accept the literal string "0", but it could be read to suggest that "kill -0000" should be accepted. i'm not sure this is worth complicating the code to fix that, given that no one would ever do it.

[ivy]

no longer needed as the problem was fixed in 3ef02a3d2e853766916a4f80e40ad8ba134b0363