timeout: print the signal number 0 instead of EXIT

timeout: print the signal number 0 instead of EXIT

POSIX.1-2024 added sig2str but leaves the behavior when called with
signal 0 unspecified. FreeBSD 15.0 does not return the signal name EXIT
like Gnulib's version causing a test failure. This fixes that and
changes the behavior to print 0 instead of EXIT, to avoid confusion when
the program does not exit.

* NEWS: Mention the change.
* src/timeout.c (cleanup): Use snprintf instead of sig2str if the signal
is 0.
* tests/timeout/timeout.sh: Updated the expected output.