pgrep: Use only --signal option for signal

pgrep: Use only --signal option for signal

When pgrep was used to match on signal, it makes sense to use
the same signal parsing code as pkill. Unfortunately the
"find the signal" part is a little too enthusaistic about what a
signal is, meaning

pgrep -u -42

fails because the signal becomes "42" and then there is no UID.

This is a bit sad for pkill but has been that way for a long
time. For pgrep this is new so now only the long form
pgrep --signal <X>
will work.

In addition, when using --signal if pgrep/pkill couldn't work
out what the signal was it just silently ignored it. It now
complains and aborts.

References:
 https://bugs.debian.org/1031765
 commit 866abacf8805a74fb7c59cae1f64963e0a540b14