1.0.43.6: fix overeager input-buffer filling by external-format routines

1.0.43.6: fix overeager input-buffer filling by external-format routines

 Fixes lp#643686.

 Previously the character-input functions returned only after filling the
 entire request by fast-read-char-refill, or if an EOF was reached.

 This meant that on a pipe we would not receive any input until there
 was a buffer's worth of it, or the other end closed.

 Not so good. New the drill is:

   0, N characters requested.

   1. Decode upto N characters from binary buffer to the character
      buffer.

   2. If any characters were decoded or at EOF, return.

   3. Otherwise refill the binary buffer with at most one read()
      and goto 1.

 Previously at #1 we returned only if the entire request was
 satisfied.