Add internal-only port structure and move iconv descriptors there.

Add internal-only port structure and move iconv descriptors there.

* libguile/ports-internal.h: New file.

* libguile/Makefile.am (noinst_HEADERS): Add ports-internal.h.

* libguile/ports.h (scm_t_port): Add a comment mentioning that the
  'input_cd' and 'output_cd' fields of the public structure are no
  longer what they seem to be.

* libguile/ports.c: Include ports-internal.h.

  (finalize_port, scm_i_remove_port, get_iconv_codepoint, get_codepoint,
  scm_i_set_port_encoding_x): Access 'input_cd' and 'output_cd' via the
  new internal port structure.

  (scm_new_port_table_entry): Allocate and initialize the internal port
  structure.

* libguile/print.c: Include ports-internal.h.

  (display_string_using_iconv, display_string): Access 'input_cd' and
  'output_cd' via 'internal' pointer.