Remove the self-upgrade check.

Fixes #2089.
Add a right-biased variant of 'NubList'.
GhcOptions: use `NubListR` where appropriate.

Some Cabal API clients apparently use `foldl mappend` on large lists of
`GhcOptions` records. Fixes #2110.
80-col violations.
Merge pull request #2111 from 23Skidoo/ghc-options-union

GhcOptions: use `NubListR` where appropriate.
Remove support for Hugs and NHC98.

Fixes #2130.
Merge pull request #2090 from 23Skidoo/no-update-check

Remove the self-upgrade check.
Merge pull request #2168 from 23Skidoo/remove-hugs-and-nhc-support

Remove support for Hugs and NHC98.
'parseInstalledPackageInfo': silently ignore 'hugs-options'.

Fixes #2171.
cabal-install : Add a 'user-config' command.

The 'user-config' command allows pseudo-diff-ing and updating of the
user's ~/.cabal/config file. The diff is against what cabal would
generated if the user config file did not exist and the update
command overlays the user's existing settings over the current
version of the default settings and writes it back to ~/.cabal/config.

Closes: #2159