Support more POSIX signals in PosixSignal

Currently a small subset of all the allowed POSIX signals are supported. In particular, useful ones like USR1 and USR2 are missing.

For Linux, you can see the list here: https://man7.org/linux/man-pages/man7/signal.7.html

The POSIX version that defines the signal is listed there as well.

I'm not entirely confident which ones make sense, but at least SIGUSR1 and SIGUSR2 would be super useful.