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

fusl/src/unistd/fchdir.c

Index: fusl/src/unistd/fchdir.c
diff --git a/fusl/src/unistd/fchdir.c b/fusl/src/unistd/fchdir.c
new file mode 100644
index 0000000000000000000000000000000000000000..72c3915eee095964e0f161183ccfcff28862a47a
--- /dev/null
+++ b/fusl/src/unistd/fchdir.c
@@ -0,0 +1,17 @@
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include "syscall.h"
+
+void __procfdname(char *, unsigned);
+
+int fchdir(int fd)
+{
+	int ret = __syscall(SYS_fchdir, fd);
+	if (ret != -EBADF || __syscall(SYS_fcntl, fd, F_GETFD) < 0)
+		return __syscall_ret(ret);
+
+	char buf[15+3*sizeof(int)];
+	__procfdname(buf, fd);
+	return syscall(SYS_chdir, buf);
+}