Porters: new @extra functionality in pkg_*

[Marc Espie <espie@nerim.net>]

I committed an update to pkg_* that adds recognition for an @extra keyword.
That keyword is used in conjunction with -c in pkg_delete: to allow extra
files to get deleted. There's also a second related keyword @extraunexec,
whose purpose you can guess.

DEINSTALL scripts will get notified by the env variable PKG_DELETE_EXTRA
being set to Yes, instead of No.

The intent is to allow configuration files to get removed on demand, by
using pkg_delete -c.

This is very useful for bulk package building, where you usually want to
start on a `clean' system, and hence want to remove every configuration
file from a previous run.

Please check your ports and create updates that contain @extra annotations.