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

fusl/src/signal/sigset.c

+#include <signal.h>
+
+void (*sigset(int sig, void (*handler)(int)))(int)
+{
+        struct sigaction sa, sa_old;
+        sigset_t mask;
+
+        sigemptyset(&mask);
+        if (sigaddset(&mask, sig) < 0)
+                return SIG_ERR;
+        
+        if (handler == SIG_HOLD) {
+                if (sigaction(sig, 0, &sa_old) < 0)
+                        return SIG_ERR;
+                if (sigprocmask(SIG_BLOCK, &mask, &mask) < 0)
+                        return SIG_ERR;
+        } else {
+                sa.sa_handler = handler;
+                sa.sa_flags = 0;
+                sigemptyset(&sa.sa_mask);
+                if (sigaction(sig, &sa, &sa_old) < 0)
+                        return SIG_ERR;
+                if (sigprocmask(SIG_UNBLOCK, &mask, &mask) < 0)
+                        return SIG_ERR;
+        }
+        return sigismember(&mask, sig) ? SIG_HOLD : sa_old.sa_handler;
+}