portindex should support not reindexing ports that don't need it

[ryandesign@…]

The portindex script takes several minutes to reindex all of our ports, and this will only get worse as the number of ports grows. Often all that one wants to do is add a new port to the index, or update a single port's entry in the index. portindex should provide a way to accomplish that more quickly than rebuilding the entire index.

This was initially  suggested on macports-dev by Joshua Root.

[dluke@…]

A long time ago, this was going to be a feature added along with converting the portindex to a sqlite database.

[jmr@…]

While not what this ticket originally asked for, r66845 makes running portindex fast enough that it doesn't matter.