Add detailed breakdown of what maintainer scripts can assume

Add detailed breakdown of what maintainer scripts can assume

For each possible action in a maintainer script, state what the
maintainer script can assume in that case.  Remove references to
using Pre-Depends for controlling what's available during package
removal.  Be clear that no postrm actions can rely on dependencies
being available.  Remove references saying that prerm can rely on
dependencies being configured.

Based on extensive analysis by Ian Jackson.