Change all multi-word options to dashes instead of underscores.

Change all multi-word options to dashes instead of underscores.

Currently some of these options use dashes to separate the words
while others use the underscore character instead. The auto-generated
--detailed-help option contains a dash, so the only consistent way
is to use dashes everywhere.

Hence this commit replaces all underscores in option names by dashes
and updates the documentation accordingly.

Fortunately, this does not require to change the code, because
gengetopt translates dashes into underscores in the generated C code.
It is, however, an API change, so this is 0.5.0 material.