fix signalfd not to ignore flags

also include fallback code for broken kernels that don't support the
flags. as usual, the fallback has a race condition that can leak file
descriptors.

src/linux/signalfd.c

 #include <sys/signalfd.h>
 #include <signal.h>
+#include <errno.h>
+#include <fcntl.h>
 #include "syscall.h"
 
 int signalfd(int fd, const sigset_t *sigs, int flags)
 {
-	return syscall(SYS_signalfd, fd, sigs, _NSIG/8);
+	int ret = __syscall(SYS_signalfd4, fd, sigs, _NSIG/8, flags);
+	if (ret != -ENOSYS) return __syscall_ret(ret);
+	ret = __syscall(SYS_signalfd, fd, sigs, _NSIG/8);
+	if (ret >= 0) {
+		if (flags & SFD_CLOEXEC)
+			__syscall(SYS_fcntl, ret, F_SETFD, FD_CLOEXEC);
+		if (flags & SFD_NONBLOCK)
+			__syscall(SYS_fcntl, ret, F_SETFL, O_NONBLOCK);
+	}
+	return __syscall_ret(ret);
 }