1.0.43.52: correct char-size for :EXTERNAL-FORMAT :DEFAULT

1.0.43.52: correct char-size for :EXTERNAL-FORMAT :DEFAULT

 Fixes bug 657183.

 Make picking the char-size part of picking input/output routines,
 and make set-fd-stream-routines set FD-STREAM-CHAR-SIZE from it.

 For cleanliness sake, don't ever construct an FD-STREAM with an
 inconsistent external-format and char-size -- meaning the default
 external-format for the FD-STREAM structure cannot be :DEFAULT.
 ...this should not matter, but in case someone inspects an unfinished
 stream instance, at least things make a bit more sense.