Per the Linux [man page](https://man7.org/linux/man-pages/man2/sigprocmask.2.html), `sigprocmask(2)` is unspecified in a multithreaded process, perhapss we should make it `unsafe` due to this reason. Or we should simply remove it considering that `pthread_sigmask()` is basically the safe version of `sigprocmask()`.
