fish: small refactor of config reading code

Even though so far there is just one possible setting to read, isolate
in an own function the code to parse a configuration file and read the
settings out of it.

Now there's a new config_t handle used every time, but since config_read
would reset an handle completely, there is no behaviour change.
fish: change order of config files being read

First read the global configuration and then the local one in user's
HOME, so the latter can really override system settings.