don't reject unknown/future flags in sigaltstack, allow SS_AUTODISARM

historically, and likely accidentally, sigaltstack was specified to
fail with EINVAL if any flag bit other than SS_DISABLE was set. the
resolution of Austin Group issue 1187 fixes this so that the
requirement is only to fail for SS_ONSTACK (which cannot be set) or
"invalid" flags.

Linux fails on the kernel side for invalid flags, but historically
accepts SS_ONSTACK as a no-op, so it needs to be rejected in userspace
still.

with this change, the Linux-specific SS_AUTODISARM, provided since
commit 9680e1d0 but unusable due to
rejection at runtime, is now usable.

src/signal/sigaltstack.c

@@ -9,7 +9,7 @@ int sigaltstack(const stack_t *restrict ss, stack_t *restrict old)
 			errno = ENOMEM;
 			return -1;
 		}
-		if (ss->ss_flags & ~SS_DISABLE) {
+		if (ss->ss_flags & SS_ONSTACK) {
 			errno = EINVAL;
 			return -1;
 		}