include/opt: Use an associative array to simplify flag parsing

include/opt: Use an associative array to simplify flag parsing

When parsing the flags of an option, the opt module uses a case
construct, which looks clumsy, isn't very easy to read, and makes
adding new flags more complicated than need be.

This commit modifies the opt module to use an associative array
to simplify flag parsing. This way, the parser only has to use a
flag as index into the array in order to find out its internal
value.