read_builtin_tables() wanted to run in builtindir, and
read_custom_tables() wanted to run in configdir. Bothersome. Use new
fopenat() to relax those requirements.
The chdir() satisfying them are now superflous, remove them.
File names in econfig need to be interpreted relative to configdir.
This wasn't the case everywhere for keys data and info.
Fix this by changing variables gamedir and infodir to hold absolute
names. Change builtindir likewise, for consistency. Store the values
from econfig in gamedir_conf, infodir_conf and builtindir_conf.
Uses new fnameat() to derive absolute names from possibly relative
ones.
