First step along the way of having GHC understand DOS-style paths
(hot new feature!) - if ';' occurs in a -i<dirs> list, ';' is used
to separate entries on the import search path, otherwise its trusty
old ':'.

[This commit is made simply to unblock some Win32 users, but it
 shouldn't have any untoward effects on others. If it does, please
 back it out as I'm about to take off for a couple of weeks.]