refactor port encoding modes: utf-8 and iconv
* libguile/ports.h (struct scm_t_port): 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 some cases.
* libguile/ports.c (scm_c_make_port_with_encoding): Init the encoding
mode.
(scm_i_remove_port): Adapt to call close_iconv_descriptors.
(finalize_iconv_descriptors, open_iconv_descriptors):
(close_iconv_descriptors): New infrastructure to manage iconv
descriptors.
(scm_i_port_iconv_descriptors): New internal helper.
(scm_i_set_port_encoding_x): Use open_iconv_descriptors, if needed.
(get_iconv_codepoint): Use pt->iconv_descriptors.
(get_codepoint): Check the port encoding mode flags.
* libguile/print.c (display_string_using_iconv): Use
scm_i_port_iconv_descriptors.
(display_string): Use pt->encoding_mode flag.