Port conversion strategies internally are symbols

* libguile/ports.h (scm_t_port): Represent the conversion strategy as a
  symbol, to make things easier for Scheme.  Rename to
  "conversion_strategy".
  (scm_c_make_port_with_encoding): Change to take encoding and
  conversion_strategy arguments as symbols.
  (scm_i_string_failed_conversion_handler): New internal helper, to turn
  a symbol to a scm_t_string_failed_conversion_handler.
  (scm_i_default_port_encoding): Return the default port encoding as a
  symbol.
  (scm_i_default_port_conversion_strategy)
  (scm_i_set_default_port_conversion_strategy): Rename from
  scm_i_default_port_conversion_handler et al.  Take and return Scheme
  symbols.
* libguile/foreign.c (scm_string_to_pointer, scm_pointer_to_string): Use
  scm_i_default_string_failed_conversion_handler instead of
  scm_i_default_port_conversion_handler.
* libguile/print.c (PORT_CONVERSION_HANDLER): Update definition.
  (print_normal_symbol): Use PORT_CONVERSION_HANDLER.
* libguile/r6rs-ports.c (make_bytevector_input_port):
  (make_custom_binary_input_port, make_bytevector_output_port): Adapt to
  changes in scm_c_make_port_with_encoding.
* libguile/strings.h:
* libguile/strings.c (scm_i_default_string_failed_conversion_handler):
  New helper.
  (scm_from_locale_stringn, scm_from_port_stringn):
  (scm_to_locale_stringn, scm_to_port_stringn): Adapt to interface
  changes.
* libguile/strports.c (scm_mkstrport): Adapt to
  scm_c_make_port_with_encoding change.
* libguile/ports.c (scm_c_make_port): Adapt to
  scm_c_make_port_with_encoding change.
  (ascii_toupper, encoding_matches, canonicalize_encoding): Move down in
  the file.
  (peek_codepoint, get_codepoint, scm_ungetc): Adapt to port conversion
  strategy change.  Remove duplicate case in get_codepoint.
  (scm_init_ports): Move symbol initializations to the same place.

libguile/strings.h

@@ -100,6 +100,9 @@ typedef enum
 
 SCM_INTERNAL SCM scm_nullstr;
 
+SCM_INTERNAL scm_t_string_failed_conversion_handler
+scm_i_default_string_failed_conversion_handler (void);
+
 SCM_API SCM scm_string_p (SCM x);
 SCM_API SCM scm_string (SCM chrs);
 SCM_API SCM scm_make_string (SCM k, SCM chr);