Use 'infinity' instead of 'unlimited'

Use 'infinity' for consistency with other ejabberd settings.  This just
updates a record declaration, the actual code should already cope with
'infinity' (the 'max_allocs' field is currently unused, and any atom
should work for 'max_permissions').
Merge pull request #3 from weiss/infinity-vs-unlimited

Use 'infinity' instead of 'unlimited'