cells: Fix login manager login limit interpretation

Addresses an issue in FTP and DCAP doors in which the login limit
was would allow n + 1 logins when n was given as a limit. Also
addresses an issue in which a limit of zero was considered to
be no limit at all. The correct value for disabling the limit is
-1.

Target: trunk
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Christian Bernardt <christian.bernardt@desy.de>
Patch: http://rb.dcache.org/r/5676/