HEADSUP: default for ports/Makefile 'make update' is changing

[Mark Linimon]

If you use 'make update', the default method is going to become portsnap.
Currently, if you do not define one of the Makevars PORTSNAP_UPDATE,
SUP_UPDATE, or CVS_UPDATE first, the code just complains at you and
does nothing.

This is one less thing that has to be set by default by an administrator.

More advanced users can still set SUP_UPDATE or CVS_UPDATE, but if none of
these is set, portsnap is assumed.  This deprecates PORTSNAP_UPDATE.

mcl