io::ErrorKind: rationalise ordering in main enum

io::ErrorKind: rationalise ordering in main enum

It is useful to keep some coherent structure to this ordering.  In
particular, Other and Uncategorized should be next to each other, at
the end.

Also it seems to make sense to treat UnexpectedEof and OutOfMemory
specially, since they are not like the other errors (despite
OutOfMemory also being generatable by some OS errors).

So:
 * Move Other to the end, just before Uncategorized
 * Move Unsupported to between Interrupted and UnexpectedEof
 * Add some comments documenting where to add things

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>