wftk core: user directory adaptor

A user directory adaptor is responsible for interfacing with a directory system in order to define users and groups, determine authorization of a user given username and password, determine group membership, and I'm not sure what else at the moment.

These are the functions exposed by a user directory adaptor:

Function Parameters Purpose

0: init Initialize adaptor instance

1: free Clean up instance before shutting down

2: info Return XML describing the adaptor

3: get userid Retrieve user description

4: auth XML * user
password Return user if authenticated, NULL if not.

Implementations:

USER_localxml (stores XML files in local directory)
(TODO) USER_ldap.html (asks an LDAP directory for user info)

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Function	Parameters	Purpose
0: init		Initialize adaptor instance
1: free		Clean up instance before shutting down
2: info		Return XML describing the adaptor
3: get	userid	Retrieve user description
4: auth	XML * user password	Return user if authenticated, NULL if not.