Showstopper and regression against 5.0.24.

Previously, we ignored seek() errors (see Bug#22828) and let seek()s
against pipes fail.  Now, since we check that a seek didn't fail,
and return without reading, this bug popped up.

This restores the behavior for file-ish objects that could never be 
seek()ed.


mysys/mf_iocache.c:
  If we detect early that the file is not tell()able, then we should
  assume that it's also not seek()able and therefore we should never
  set the (poorly named) "seek_not_done" flag so that we don't immedi-
  ately try to seek() when reading later.
  
  The problem was that tell() was returning -1, so when we read later, 
  we needlessly tried to seek to position  (unsigned long) -1 . 
  
  Also, if we think we're supposed to seek to a position in a file and 
  the file is un-tell()able, then abort.