Add an --install-method=symlink|copy flag which specifies how to perform
the installation.

* --symlink-bindir is now gone, replaced by --installdir
* --install-method=copy is useful in Windows where symlinking is not
supported

All new-install flags can now be configured in ~/.cabal/config

* NewInstallFlags changed to ClientInstallFlags (more descriptive than
InstallExFlags (like ConfigExFlags))
* ClientInstallFlags is now part of SavedConfig