Auth/LDAP: add get_{users, groups} subs for syncing

Auth/LDAP: add get_{users, groups} subs for syncing

this adds the subs which actually query the LDAP for users/groups
and returns the value in format which makes it easy to insert
in our parsed user.cfg

when we find a user/groupname which cannot be in our config,
we warn the verification error

for groups, we append "-$realm" to the groupname, to lower the chance of
accidental overwriting of existing groups (this will be documented
in the api call since it technically does not prevent overwriting, just
makes it more unlikely)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>