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