mirbsd: add some more support * m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): MirBSD is in BSD family. * m4/gc-random.m4 (gl_GC_RANDOM): MirBSD supports same random devices as OpenBSD. * m4/host-os.m4 (mirbsd): Add MirBSD. Signed-off-by: Eric Blake <eblake@redhat.com>