portmaster: print /usr/ports/UPDATING on update

[David Demelier]

Hi,

A lot of people always forget to read UPDATING (that's normal we'll are 
humans).

Each entry in UPDATING is like "AFFECTS: users of net-mgmt/flowd" so if 
an update of net-mgmt/flowd is available and a *recent* entry in 
UPDATING talks about then print the message.

This can prevent a lot of breakage and useless noise on lists. What do 
you think ?

Merry Christmas and happy holidays !

David.