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

fusl/src/signal/signal.c

+#include <signal.h>
+#include "syscall.h"
+#include "libc.h"
+
+int __sigaction(int, const struct sigaction *, struct sigaction *);
+
+void (*signal(int sig, void (*func)(int)))(int)
+{
+        struct sigaction sa_old, sa = { .sa_handler = func, .sa_flags = SA_RESTART };
+        if (__sigaction(sig, &sa, &sa_old) < 0)
+                return SIG_ERR;
+        return sa_old.sa_handler;
+}
+
+weak_alias(signal, bsd_signal);
+weak_alias(signal, __sysv_signal);