ports-mgmt/portsnap: Resurrect

portsnap will be removed from the base from 14.0-RELEASE onwards. To
facilitate usage of portsnap from 14.0-RELEASE onwards extract portsnap
from base and create seperate port.

Portsnap is a system for securely updating the ports tree by
distributing signed compressed snapshots. This is the client
half of that system; it downloads compressed snapshots into
/usr/local/portsnap ("portsnap fetch") and uses those to extract
a ports tree into /usr/ports ("portsnap extract") or update an
existing tree ("portsnap update").

In addition to operating entirely over HTTP, portsnap can use under
a tenth of the bandwidth required by CVSup if a copy of the ports
tree is being updated every few days.

Approved by: portmgr