Have string ports honor `%default-port-encoding'.

Have string ports honor `%default-port-encoding'.

* libguile/strports.c (scm_i_mkstrport): Remove.
  (scm_mkstrport): Don't change the port's encoding to UTF-8; convert
  STR to the default port encoding.
  (scm_strport_to_string): Fix documentation & indentation.

* libguile/strports.h (scm_i_mkstrport): Remove.

* test-suite/lib.scm (exception:encoding-error): New variable.
  (format-test-name): Set `%default-port-encoding' to "UTF-8".

* test-suite/tests/ports.test ("string ports")["%default-port-encoding
  is honored", "suitable encoding [latin-1]", "suitable encoding
  [latin-3]", "wrong encoding"]: New tests.

* test-suite/tests/r6rs-ports.test ("7.2.11 Binary
  Output")["put-bytevector with UTF-16 string port", "put-bytevector
  with wrong-encoding string port"]: New tests.

* test-suite/tests/reader.test (read-string): Set
  `%default-port-encoding' to `#f'.
  ("reading")["unprintable symbol"]: Use a string that doesn't contain
  zeros.

* doc/ref/api-io.texi (String Ports): Document encoding issues with
  `call-with-output-string' and `with-output-to-string'.