sysutils/freebsd-snapshot: allows to zfs filesystems to have unlimited snapshots

The periodic-snapshot supplied with the sysutils/freebsd-snapshot port
limits the total number of snapshots per file system to 20, which is
correct for ufs filesystems but does not apply to zfs which can have 2^64.

PR: 215828
Submitted by: hostmaster@GTS.NET
Approved by: maintainer timeout