Changed all paths in the INSTALL_SCHEMES dict to Unix syntax, and added

Changed all paths in the INSTALL_SCHEMES dict to Unix syntax, and added
'convert_paths()' method to convert them all to the local syntax (backslash
or colon or whatever) at the appropriate time.

Added SCHEME_KEYS to get rid of one hard-coded list of attributes (in
'select_scheme()').

Default 'install_path_file' to true, and never set it false (it's just
there in case some outsider somewhere wants to disable installation of the
.pth file for whatever reason).

Toned down the warning emitted when 'install_path_file' is false, since we
no longer know why it might be false.

Added 'warn_dir' flag to suppress warning when installing to a directory
not in sys.path (again, we never set this false -- it's there for outsiders
to use, specifically the "bdist_*" commands).

Pulled the loop of 'change_root()' calls out to new method 'change_roots()'.

Comment updates/deletions/additions.