Have `scm_getc' honor the port's conversion strategy.

Have `scm_getc' honor the port's conversion strategy.

* libguile/ports.c (get_codepoint): Reset `pt->input_cd' upon failure.
  If `pt->ilseq_handler' is `SCM_ICONVEH_QUESTION_MARK', then return a
  question mark.
  [failure]: Use `scm_encoding_error' when raising an error.

* test-suite/lib.scm (exception:encoding-error): Adjust regexp.

* test-suite/tests/ports.test ("string ports")["read-char, wrong
  encoding, error", "read-char, wrong encoding, escape", "read-char,
  wrong encoding, substitute"]: New tests.