Fix for LDAP Web INterface auth

Problem description

When integrating Observium with LDAP that has strong requirement of group nesting and usage of "groupOfNames", ran into following issues:

• When groups are recursively fetched from LDAP (i.e. uid=admin is member of cn=system_admins, and cn=system_admins is member of cn=observium_admins which is used to get privileges in Observium interface), upon 2nd iteration I received not "cn=system_admins", but just "c" this is due to a fact that ldap_search_user function referenced "$element[$config['auth_ldap_attr']['dn']][0]" instead of just "$element[$config['auth_ldap_attr']['dn']]";
• When group search is performed ldap_filter_create function uses fixed value of "group" for objectClass, but the LDAP I integrated with uses "groupOfNames".

Proposed solution

Use not "$element[$config['auth_ldap_attr']['dn']]" and not "$element[$config['auth_ldap_attr']['dn']][0]" to fix recursive group search.
Introduce "$config['auth_ldap_attr']['group']" variable in config to fix different names of group attribute in LDAP. Use default value of "group" for this variable to avoid breaking existing installations.

Attached patch fixes both of them.