Support for using $u username variable in AFP volume definitions

* Prepare CNID backend dbd for user variable expansion
* Store a struct vol pointer in the CNID db handles.
* Pass three arguments to the CNID daemon:
  - the afp.conf volume section name
  - the volume path
  - an optional username

* Rename several struct elements by prefixing them with a struct tag,
  helps catching bad or missing assignments.

* Disable uniconv, doesn't use laod_volumes() and thus wont work with
  the new CNID dbd backend.

From FR#90.

Signed-off-by: Ralph Boehme <rb@sernet.de>
dbd: two fixes for new user option

* initialize user variable with NULL
* add -u to option summary line

Signed-off-by: Ralph Boehme <rb@sernet.de>