loadconfig: add and use for items, names, planets, ports and ships

Add the master configuration file yastg.conf and parsing of that file in
loadconfig. Use that code for loading items, names, planets, ports and
ships from the files specified in yastg.conf, instead of from hardcoded
paths relative to the current directory.

Configuration files are found using libxdg to try to keep in line with
modern standard. Add this library as a dependency in autotools.
civ: move civ data structure into universe

Previously, all data regarding civilizations was kept in a separate
structure. Move that structure into the universe struct, in the same way
as all other data.
civ: load civs properly in loadconfig

Use the master configuration file to specify civilizations to be loaded,
and use loadconfig to do so. Move all civilization data from civs/ to
data/, as they should now be in the same directory.
ptrarray: use parseconfig in ptrarray_to_file()

It makes more sense to use the already written parseconfig routines to
load the files. This also requires updating all data files containing
names and enclosing all names in double quotes.
constellation: load via yastg.conf

constellations are now loaded using the same syntax in yastg.conf as the
other data. Reuse the name infrastructure for this, as it seems to be
adequately suited and we won't have to duplicate code.
names: add is_names_loaded()

This function can be used to query if a name list has had anything
loaded into it or not.
loadconfig: add config sanity check

Sanity check the configuration after loading everything to make sure all
the data needed has really been loaded ok.
refactor load functions to remove global universe

Rename load functions to from load_all_foo() to load_foo_from_file(), to
better reflect the new usage. Add the universe struct as a parameter to
them instead of passing it as a global.
map: add const where applicable
clean up includes