Add proj= parameter to [shpext], [mapext], etc. template tags.

[jimk]

I recently ran into situation where it would be useful to be able to place [shpext proj=...] in a template much like can be done with [shpxy]. When looking through the code, I see this isn't a new idea because there is a line listing this as a TODO item.

The changes are all in maptemplate.c. I submitted a patch to Steve who cleaned it up and committed it as r8530.