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

fusl/src/thread/pthread_mutex_unlock.c

+#include "pthread_impl.h"
+
+int __pthread_mutex_unlock(pthread_mutex_t *m)
+{
+        pthread_t self;
+        int waiters = m->_m_waiters;
+        int cont;
+        int type = m->_m_type & 15;
+        int priv = (m->_m_type & 128) ^ 128;
+
+        if (type != PTHREAD_MUTEX_NORMAL) {
+                self = __pthread_self();
+                if ((m->_m_lock&0x7fffffff) != self->tid)
+                        return EPERM;
+                if ((type&3) == PTHREAD_MUTEX_RECURSIVE && m->_m_count)
+                        return m->_m_count--, 0;
+                if (!priv) {
+                        self->robust_list.pending = &m->_m_next;
+                        __vm_lock();
+                }
+                volatile void *prev = m->_m_prev;
+                volatile void *next = m->_m_next;
+                *(volatile void *volatile *)prev = next;
+                if (next != &self->robust_list.head) *(volatile void *volatile *)
+                        ((char *)next - sizeof(void *)) = prev;
+        }
+        cont = a_swap(&m->_m_lock, (type & 8) ? 0x40000000 : 0);
+        if (type != PTHREAD_MUTEX_NORMAL && !priv) {
+                self->robust_list.pending = 0;
+                __vm_unlock();
+        }
+        if (waiters || cont<0)
+                __wake(&m->_m_lock, 1, priv);
+        return 0;
+}
+
+weak_alias(__pthread_mutex_unlock, pthread_mutex_unlock);