[apt-pkg] yet another bit of mmap and pkgcachegen housekeeping

[apt-pkg] yet another bit of mmap and pkgcachegen housekeeping
* add mmap error message also to the dynamic mmap
* remove some more {Ver,Desc} == 0 checks in for loops
* try to respect the given flags to the dynamic mmap
* open cached caches not as ReadOnly and not as Shared, so we
  always have a copy of the cache in the memory we can modify
  (e.g. set the hold state on-the-fly)