Allow to validate the config file without X.

Allow to validate the config file without X.

We're going to call parse_configuration() very early if -C is given on
the command line. Instead of the previous "only_check_config", which has
been a global variable, we now simply pass use_nagbar as false if we're
just validating.

This causes the whole parsing to run without X and of course without
starting nagbar and displaying the errors to standard out/error instead.

The return code of parse_configuration() is now a boolean which
represents whether an error occured during parsing and the programs exit
code is returned accordingly.

Although the config parser still has a lot of side-effects, we now can
parse without the need to have an XCB connection. A nicer implementation
would be to just set the new font and load it just after we're done
parsing, but to ensure we don't break functionality we just load a dummy
FONT_TYPE_NONE if XCB isn't available. The main reason for going this
route is that it's a bit difficult to test fonts in a distribution
agnostic way without bundling fonts with i3 (or Xdummy to be more
exact).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>