Fix encoding errors with strings returned by string ports

Fix encoding errors with strings returned by string ports

String ports, being 8-bit, store strings using the character encoding
of the port.  This fixes a bug where the default character encoding, and
not the port's encoding, was being used to convert the string port data
back to a string.

* libguile/strports.c: extra comments
  (scm_strport_to_string):  use port's encoding when converting port data
  to a string

* libguile/strings.c (scm_i_from_stringn): renamed from scm_from_stringn
  and made internal.  All callers changed.
  (scm_from_stringn): renamed to scm_i_from_stringn.

* libguile/strings.h: declaration for scm_i_from_stringn