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

fusl/src/unistd/fchown.c

+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include "syscall.h"
+
+void __procfdname(char *, unsigned);
+
+int fchown(int fd, uid_t uid, gid_t gid)
+{
+        int ret = __syscall(SYS_fchown, fd, uid, gid);
+        if (ret != -EBADF || __syscall(SYS_fcntl, fd, F_GETFD) < 0)
+                return __syscall_ret(ret);
+
+        char buf[15+3*sizeof(int)];
+        __procfdname(buf, fd);
+#ifdef SYS_chown
+        return syscall(SYS_chown, buf, uid, gid);
+#else
+        return syscall(SYS_fchownat, AT_FDCWD, buf, uid, gid);
+#endif
+
+}