parse_user_cfg: correctly parse group names in ACLs

parse_user_cfg: correctly parse group names in ACLs

usernames are allowed to start with '@', so adding a user '@test@pve'
and adding it to an ACL should work, instead of ignoring that part of
the ACL entry.

So use verify_groupname to additionally enforce that the group name we
extracted does not include an additional @, as then it cannot be a
group.

note: there is no potential for user and group to be confused, since a
username must end with '@REALM', and a group reference in an ACL can
only contain one '@' (as first character).

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>