The ConfigFlags now contain two different sets of install dirs, one for global and one for user installs.
This simplifies the install dirs handling a lot, and allows the user to specify both user and global install dirs in the config file, and chose installation type on the command line.
We now ignore the config file and command-line compiler flavor when choosing install directories, since that
is so messy. This might break installs with Hugs, but that requires testing.
The old code didn't work when the compiler was specified on the command-line anyway.