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

fusl/src/misc/forkpty.c

+#include <pty.h>
+#include <utmp.h>
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/wait.h>
+#include <pthread.h>
+
+int forkpty(int *pm, char *name, const struct termios *tio, const struct winsize *ws)
+{
+        int m, s, ec=0, p[2], cs;
+        pid_t pid=-1;
+        sigset_t set, oldset;
+
+        if (openpty(&m, &s, name, tio, ws) < 0) return -1;
+
+        sigfillset(&set);
+        pthread_sigmask(SIG_BLOCK, &set, &oldset);
+        pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+
+        if (pipe2(p, O_CLOEXEC)) {
+                close(s);
+                goto out;
+        }
+
+        pid = fork();
+        if (!pid) {
+                close(m);
+                close(p[0]);
+                if (login_tty(s)) {
+                        write(p[1], &errno, sizeof errno);
+                        _exit(127);
+                }
+                close(p[1]);
+                pthread_setcancelstate(cs, 0);
+                pthread_sigmask(SIG_SETMASK, &oldset, 0);
+                return 0;
+        }
+        close(s);
+        close(p[1]);
+        if (read(p[0], &ec, sizeof ec) > 0) {
+                int status;
+                waitpid(pid, &status, 0);
+                pid = -1;
+                errno = ec;
+        }
+        close(p[0]);
+
+out:
+        if (pid > 0) *pm = m;
+        else close(m);
+
+        pthread_setcancelstate(cs, 0);
+        pthread_sigmask(SIG_SETMASK, &oldset, 0);
+
+        return pid;
+}