Add a "fork" of musl as //fusl.

fusl/src/linux/signalfd.c

Index: fusl/src/linux/signalfd.c
diff --git a/fusl/src/linux/signalfd.c b/fusl/src/linux/signalfd.c
new file mode 100644
index 0000000000000000000000000000000000000000..4bf43326fe5a7ee55858e728cf5bcff07bb64667
--- /dev/null
+++ b/fusl/src/linux/signalfd.c
@@ -0,0 +1,21 @@
+#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)
+{
+	int ret = __syscall(SYS_signalfd4, fd, sigs, _NSIG/8, flags);
+#ifdef SYS_signalfd
+	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);
+	}
+#endif
+	return __syscall_ret(ret);
+}