Search for config files in "cfg"

[gak]

After building with Makefile.linux (might apply to other build methods), the cfg directory is not looked at when the binary is executed from the root of the project. A fix for it is to copy gravit.cfg down a directory into the root.

Gravit itself should search for gravit.cfg in cfg/

[FrMo]

works for me on windows, and also when doing

./autogen.sh; ./configure && make install

on linux; so it is probably related to building with Makefile.linux.
I have actually changed the default location to cfg/ to get the windows install working. Maybe we can apply the same to other platforms.

See changes to command.c and gravit.h in commit 6dc9f07