bug#11612: 24.1; Defaults create warning on startup

[Drew Adams]

> Given that in Windows 7 writing to c:\ requires administrative
> priviledges, perhaps we should be more strongly discouraging setting
> HOME to c:\ for users who are blindly copying obsolete 
> instructions they find on the internet.

Why?  What is the consequence if an Emacs user (using admin privileges) sets
HOME to c:\?  Not a rhetorical question - I know little to nothing about this.

_If_ (dunno) there is no negative consequence in terms of _using_ Emacs, and the
only effective consequence is that Windows 7 will notify a user that s?he must
have admin privileges to _set_ HOME to c:\, is that a problem?

In any case, if there is a downside then the "discouragement" or recommendation
should explicitly point out the situation - e.g. list potential problems or
point (via URL) to such a list of problems.  We should not just discourage c:\
for all Windows users, or even for Windows 7 users only, without giving some
info about why etc.