sigaltstack: enforce dynamic MINSIGSTKSZ limit

commit 996b6154b20184c3b08cce28eb01edb7f47e9413 added support for
querying the dynamic limit but did not enforce it in sigaltstack. the
kernel also does not seem to reliably enforce it, or at least does not
necessarily enforce the same limit exposed to userspace, so it needs
to be enforced here.

src/signal/sigaltstack.c

@@ -1,11 +1,13 @@
 #include <signal.h>
 #include <errno.h>
+#include <unistd.h>
 #include "syscall.h"
 
 int sigaltstack(const stack_t *restrict ss, stack_t *restrict old)
 {
 	if (ss) {
-		if (!(ss->ss_flags & SS_DISABLE) && ss->ss_size < MINSIGSTKSZ) {
+		size_t min = sysconf(_SC_MINSIGSTKSZ);
+		if (!(ss->ss_flags & SS_DISABLE) && ss->ss_size < min) {
 			errno = ENOMEM;
 			return -1;
 		}