added auth_isMember()

This function abstracts checking a given user and her groups against a
given member list (as used in the superuser and manager options).

It is also used in auth_isManager() and auth_isAdmin(), unlike the
previous function, this one skips the nameencode step as it should be
unnessary here (all input is given decoded).

The test cases where extended by some non-ID user and group names.

People with non-plain auth backends should check that their
administrator and manager setups still work as expected