util: improve the command line argument parsing in wmmenugen

util: improve the command line argument parsing in wmmenugen

The code ignored the last argument provided on the command line;

It did not support the recommended '--version' and '--help' from GNU which
are often handy;

It used an unusual syntax to specify the parser, now it can also use more
usual "-parser=name" and "-parser name", the old syntax is still supported
to avoid breaking compatibility;

When a problem is found with an argument, the program stops instead of
printing an error message and continuing;

There's been updates on the exit codes in case of problem because it is an
information that could be useful for people calling the program from a
script;

Took the opportunity to provide more information in the man page and to
get it cross-checked against the program's option list during "make check".

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>