Refactor port encoding modes: utf-8 and iconv

Refactor port encoding modes: utf-8 and iconv

Based on 6c98257f2ead0855f218369ea7f9a823cdb9727e by Andy Wingo.

* libguile/ports-internal.h (struct scm_port_internal): Add a flag
  for the port encoding mode: UTF8 or iconv.  The iconv descriptors
  are now in a separate structure so that we can avoid attaching
  finalizers to the ports themselves in the future.
  (enum scm_port_encoding_mode): New enum.
  (struct scm_iconv_descriptors): New struct.
  (scm_i_port_iconv_descriptors): Add prototype.

* libguile/ports.c (finalize_port): Don't close iconv descriptors here.
  (scm_new_port_table_entry): Adapt to the iconv descriptors being
  moved.  Initialize 'encoding_mode'.
  (scm_i_remove_port): Adapt to call 'close_iconv_descriptors'.
  (close_iconv_descriptors): New static function.
  (get_iconv_codepoint): Use 'scm_i_port_iconv_descriptors'.
  (get_codepoint): Check the port 'encoding_mode'.
  (finalize_iconv_descriptors, open_iconv_descriptors,
  close_iconv_descriptors, scm_i_port_iconv_descriptors): New static
  functions.
  (scm_i_set_port_encoding_x): Adapt to iconv descriptors being moved
  to separate structure, to set the 'encoding_mode' flag, and to use
  'open_iconv_descriptors' and 'close_iconv_descriptors'.

* libguile/print.c (display_string_using_iconv): Use
  'scm_i_port_iconv_descriptors'.
  (display_string): Use 'encoding_mode' flag.