Don't include libunistring headers in Guile public headers

Don't include libunistring headers in Guile public headers

This requres the creation of a new type
scm_t_string_failed_conversion_handler to replace libunistring's
enum iconveh_ilseq_handler.

* libguile/strings.h: don't include <uniconv.h>
(scm_t_string_failed_conversion_handler): new enum type
(SCM_FAILED_CONVERSION_ERROR, SCM_FAILED_CONVERSION_QUESTION_MARK):
(SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE): new enum type values

* libguile/strings.c (scm_to_stringn): now takes type
scm_t_string_failed_conversion_handler.  All callers changed.

* libguile/print.c: include <uniconv.h>

* libguile/ports.c (scm_lfwrite_substr): use
scm_t_string_conversion_handler's constants

* libguile/gen-scmconfig.c (SCM_ICONVEH_ERROR):
(SCM_ICONVEH_QUESTION_MARK, SCM_ICONVEH_ESCAPE_SEQUENCE): store
iconveh_ilseq_hander constants as #define's