gui: Unify config file parsing.

The gui config file is parsed on startup, when the reload command
is executed or when SIGUSR1 is received. In all cases we set up a
gengetopt params structure and pass its address to the gengetopt
command line parser.

This commit moves the common code to the new parse_config_file_or_die()
which does the right thing for both the startup and the reload
case. The new function also contains the logic of configfile_exists(),
so this latter function can be removed.