Fix broken interaction between readline and Unicode

Fix broken interaction between readline and Unicode

This requires separate small fixes.

Readline has internal logic to deal with multi-byte characters, so
it wants bytes, not characters.

scm_c_read gets called by the vm when readline is activated, and it was
truncating multi-byte characters because soft ports didn't have the
UCS-4 capability.

Soft ports need the capability to read UCS-4 characters.  Since soft ports
may have a single byte buffer, full characters need to be stored into the
pushback buffer.

This broke the optimizations in scm_c_read for using an alternate buffer
for single-byte-buffered ports, because the opimization wasn't expecting
anything in the pushback buffer.

* libguile/vports.c (sf_fill_input): store complete chars, not single bytes

* libguile/ports.c (scm_c_read): don't use optimized path for non Latin-1.
  Add debug prints.

* libguile/string.h: make scm_i_from_stringn and scm_i_string_ref public
  so that readline can use them

* guile-readline/readline.c: read bytes, not complete chars, from the
  input port.  Convert output to the output port's locale