conf: add config-file parser

conf: add config-file parser

This again refactors the whole config subsystem but this should be the
last time. We now have generic parsers for booleans and strings and don't
leak any memory, anymore. Furthermore, this adds a config-file parser so
common options can now be placed into /etc/kmscon.conf or ~/.kmscon.conf.

The config files parse the same arguments as the command-line and the
command-line always has precedence even though it is parsed first.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>