Add Flavor helpers for DEPRECATED and EXPIRATION_DATE

If a port has multiple flavors and one needs to be deprecated,
one can now write

qt4_DEPRECATED= Qt4 has been EOL since december 2015
qt4_EXPIRATION_DATE= 2019-03-15

to let users know that changes are coming.

Approved by: portmgr (rene)
Differential Revision: https://reviews.freebsd.org/D17740