rename errors.h to ierrors.h

This is in response the bug 686978. The reported conflict with errors.h 
is actually spurious, or at least has another work-around that has been 
implemented in the macos build since 8.00. However the name does seem 
over general, so we like the proposed renaming to 'ierrors.h' to match 
iapi.h.

Since this is a change to the public api, we still support the old 
errors.h by having it include ierrors.h internally. Raph and I feel 
this is safe enough for a point release.

The attached patch is based on one submitted by Alex, who did the real 
hard work. This won't go into 8.11, but if there are no objections I'll 
commit after that goes out.

  -r