Make the various places that determine the user's "home directory"

consistent.  On Unix we now always consult getpwuid(); $HOME isn't used
at all.  On Windows the code currently consults $USERPROFILE, or $HOME
if that's not defined, but I expect this will change as soon as the win32
hackers come to a consensus.  Nothing done yet about changing the file
names used underneath $USERPROFILE.