*: unify concurrent-safe update of /etc/{passwd,group,[g]shadow}
 by Tito (farmatito AT tiscali.it)

function                                             old     new   delta
update_passwd                                        743    1171    +428
bb_perror_nomsg                                        -       9      +9
find_main                                            436     444      +8
passwd_main                                         1023    1027      +4
nameval                                              202     206      +4
chpasswd_main                                        315     319      +4
bb__parsespent                                       119     117      -2
adduser_main                                         654     650      -4
addgroup_main                                        345     341      -4
sv_main                                             1228    1222      -6
deluser_main                                         173     160     -13
bb_internal_putpwent                                  69       -     -69
add_user_to_group                                    231       -    -231
del_line_matching                                    460      31    -429
------------------------------------------------------------------------------
(add/remove: 1/2 grow/shrink: 5/6 up/down: 457/-758)         Total: -301 bytes

diff --git a/include/libbb.h b/include/libbb.h
index 0b94f70..1faa9e9 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -1142,9 +1142,16 @@
  * (otherwise we risk having same salt generated)
  */
 extern int crypt_make_salt(char *p, int cnt, int rnd) FAST_FUNC;
+
 /* Returns number of lines changed, or -1 on error */
-extern int update_passwd(const char *filename, const char *username,
-			const char *new_pw) FAST_FUNC;
+#if !(ENABLE_FEATURE_ADDUSER_TO_GROUP || ENABLE_FEATURE_DEL_USER_FROM_GROUP)
+#define update_passwd(filename, username, data, member) \
+	update_passwd(filename, username, data)
+#endif
+extern int update_passwd(const char *filename,
+		const char *username,
+		const char *data,
+		const char *member) FAST_FUNC;
 
 int index_in_str_array(const char *const string_array[], const char *key) FAST_FUNC;
 int index_in_strings(const char *strings, const char *key) FAST_FUNC;