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

fusl/src/passwd/pwf.h

+#include <pwd.h>
+#include <grp.h>
+#include <shadow.h>
+#include <stdio.h>
+#include <errno.h>
+#include <stdint.h>
+#include <string.h>
+#include <limits.h>
+#include "libc.h"
+
+int __getpwent_a(FILE *f, struct passwd *pw, char **line, size_t *size, struct passwd **res);
+int __getpw_a(const char *name, uid_t uid, struct passwd *pw, char **buf, size_t *size, struct passwd **res);
+int __getgrent_a(FILE *f, struct group *gr, char **line, size_t *size, char ***mem, size_t *nmem, struct group **res);
+int __getgr_a(const char *name, gid_t gid, struct group *gr, char **buf, size_t *size, char ***mem, size_t *nmem, struct group **res);
+int __parsespent(char *s, struct spwd *sp);