Add sigsetop extensions commonly found in musl libc and glibc

These functions (sigandset, sigisemptyset, sigorset) are commonly available
in at least musl libc and glibc; sigorset, at least, has proven quite useful
in qemu-bsd-user work for tracking the current process signal mask in a more
self-documenting/aesthetically pleasing manner.

Reviewed by: bapt, jilles, pfg
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22187