And invite anyone to have a go at merging. Also re-export the types from
the GetOpt from base rather than re-defining locally. In particular this
means we gain a few instances.

The changes wrt base GetOpt are:

One change to allow us to treat "cabal --flag command" as
"cabal command --flag" e.g. "cabal -v configure" to mean
"cabal configure -v" for flags that are not recognised as global flags,
pass them on to the sub-command. See the difference in 'shortOpt'.

Line wrapping in the 'usageInfo' output, plus a more compact rendering
of short options, and slightly less padding.