Add sigsetop extensions commonly found in musl libc and glibc

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