Fix merging of ~/ pathnames.

Don't merge the device component if the resulting pathname starts with
:absolute :home. The expansion of ~ may contain it's own device,
causing problems, especially on Windows, where
(merge-pathnames "~/") => c:C:\Users\user.
More robust homedir detection on Windows.

After consulting HOME, try USERPROFILE, then HOMEDRIVE+HOMEPATH.

Fixes lp#922117.