LDAP auth

Adds simple LDAP authentification. Uses additional config:
auth_backend: use "LDAP" to active LDAP authentification
ldaphost: LDAP server hostname
ldapport: LDAP server port
ldaprdntpl: user RDN template (%login% replaced by actual login), example: uid=%login%,ou=people,dc=example,dc=org

Please rewrite the if/else statement like this:
```
    if {
        return (ldap_bind($ldapconn, $userrdn, $password));
    } else {
        logm('LDAP connection failed');
    }
```