gui: Don't exit without shutting down curses on config reload.

Currently, if a config file containing errors is being reloaded,
gui_cmdline_parser_ext() calls exit() which leaves the terminal
in an unusable state because para_gui had no chance to call
endwin() in order to reset the terminal.

Fix this flaw by instructing gengetopt to generate code that does
not exit on errors. We can still tell that the command line or the
config file contained errors by looking at the return value of the
various parsers.