Restrict unit spawning for some structures

[historic_bruno]

Currently when a unit is trained or ungarrisoned, the game's unit spawning logic checks for available spawning points all around the structure. But wall towers have a defined "inside" and "outside", and it's annoying if your units unintentionally spawn outside the city wall.

We should be able to restrict this to a more limited angle/area with some data in the entity template, and/or control it with rally points (have the spawning point begin on the side closest the rally point).