Add CAP_FCNTL to the lease file capsicum rights, and limit to
CAP_FCNTL_GETFL. Without CAP_FCNTL_GETFL, the lease file truncation
in rewrite_client_leases() will fail to trim old data when rewriting
the file with a lesser amount of data.

Reviewed by: pjd, rwatson
Approved by: jmallett (mentor)
MFC after: 1 week