Convert para_gui to lopsub.

Convert para_gui to lopsub.

This replaces the gengetopt file for para_gui by a lopsub suite and
links para_gui with -llopsub.

The new FOR_EACH_KEY_MAP() macro is introduced for iterating over
all key bindings given at the command line or in the config file.

The bulk of the change is parse_config_file_or_die() where we get rid
of the gengetopt specific hack which removed all key bindings due to
gengetopt appending the options when the config parser is called a
second time.

The patch also adds a description section to the man page of para_gui.