Free memory allocated by the option parser.

Since confuse makes a copy of strings it stores[1] and the glib
option parser allocates strings (and says in the documentation[2] one
should free the strings with 'g_free()'), we should indeed free them
to avoid leaks.

[1] http://www.nongnu.org/confuse/manual/confuse_8h.html#dd02a10fac10c9b028c1438dff28343f
[2] https://developer.gnome.org/glib/2.39/glib-Commandline-option-parser.html#GOptionEntry