Change `scm_encoding_error' to pass the port and faulty character.

Change `scm_encoding_error' to pass the port and faulty character.

* libguile/strings.c (scm_encoding_error): Remove the `from', `to', and
  `string_or_bv' parameters; add `port' and `chr'.
  (scm_to_stringn): Update accordingly.

* libguile/strings.h (scm_encoding_error): Update accordingly.

* libguile/ports.c (scm_ungetc): Update accordingly.

* libguile/print.c (iprin1, scm_write_char): Update accordingly.

* test-suite/tests/encoding-escapes.test ("display output
  errors")["ultima", "Rashomon"]: Check the arguments of
  `encoding-error'.
  ["tekniko"]: New test.

* test-suite/tests/ports.test ("string ports")["wrong encoding"]: Adjust
  to new `encoding-error' arguments.