[Q] support for the OS X keychain

[Stephen J. Turnbull]

Aidan Kehoe writes:

 >  > I couldn't figure out how to convert the string with macros of 21.4 yet
 >  > --- maybe one of the 21.4 developers could give me some advice?
 > 
 > UTF-8 is not available as a coding system by default on 21.4. One awkward
 > way to work around that would be to check for non-ASCII in the strings, and
 > only do the translation if a) there are non-ASCII characters and b)
 > Ffind_coding_system (intern ("utf-8")) gives non-nil. 

Does this actually work correctly?  That is, HFS+ will use a NK?FD
string for the file name.  I think the file system code will normalize
if you pass it any valid UTF-8 string (this needs to be checked), but
XEmacs will not be able to round-trip.  We should check that that will
not cause problems.

I think the right thing to do here would be to simply not allow
non-ASCII strings, since this is an optional feature.  This is
obviously suboptimal, but I don't think it's a good idea to try too
hard until somebody actually asks for it.

One option would be to provide a hook on which the user could hang a
function to handle their own usage.  We could provide a sample hook
function which does the conversion for ISO 8859/1 (which is pretty
trivial).