cppcheck: avoid a possible NULL pointer dereference
refactor the way sector names are displayed

Almost entirely removed letters, they only remain for anomalies where
it's kind of a nice feature. Coords are displayed in the form (x|y).
Coords are now 0-based, same as in the data saved in the code, so most
top-left is (0|0) and most bottom-right is (width-1|height-1).
Ship target data is displayed more conveniently in the form <name> (x|y).
Target data of friendly ships is displayed as well (needed: affiliation
with the owner).
refactor station getting from CMapTile

The same data was also being held in the ships member, so get rid
of m_Outpost and m_Starbase. Savegames imcompatible.
use present stations in the generation of sector names

Possible due to previous commit. Systems get priority over stations,
but better a station name than something generic.
simplify enum with only 2 values into bool (again)