String ports use UTF-8; ignore %default-port-encoding.

* libguile/strports.c (scm_mkstrport): Use UTF-8; ignore
  %default-port-encoding.  Rename 'str_len' and 'c_pos' to
  'num_bytes' and 'c_byte_pos'.  Interpret 'pos' argument
  as a character index instead of a byte index.

* module/ice-9/boot-9.scm (%cond-expand-features): Add srfi-6 to the
  list of core features.

* module/srfi/srfi-6.scm (open-input-string, open-output-string): Simply
  re-export these, since the core versions are now compliant.

* doc/ref/api-io.texi (String Ports): Remove text that describes
  non-compliant behavior of string ports with regard to encoding.

* doc/ref/srfi-modules.texi (SRFI-0): Add srfi-6 to the list of
  core features.
  (SRFI-6): Remove text that mentions non-compliant behavior of
  core string ports.

* module/ice-9/format.scm (format):
* module/ice-9/pretty-print.scm (truncated-print):
* module/rnrs/io/ports.scm (open-string-input-port,
  open-string-output-port):
* test-suite/test-suite/lib.scm (format-test-name):
* test-suite/tests/chars.test ("combining accent is pretty-printed",
  "combining X is pretty-printed"):
* test-suite/tests/ecmascript.test (eread, eread/1):
* test-suite/tests/rdelim.test:
* test-suite/tests/reader.test (read-string):
* test-suite/tests/regexp.test:
* test-suite/tests/srfi-105.test (read-string): Don't set
  %default-port-encoding before creating string ports.

* benchmark-suite/benchmarks/ports.bm (%latin1-port): Use
  'set-port-encoding!' to set the string port encoding.
  (%utf8/ascii-port, %utf8/wide-port, "rdelim"): Don't set
  %default-port-encoding before creating string ports.

* test-suite/tests/r6rs-ports.test ("lookahead-u8 non-ASCII"): Don't set
  %default-port-encoding before creating string ports.
  ("put-bytevector with UTF-16 string port", "put-bytevector with
  wrong-encoding string port"): Use 'set-port-encoding!' to set the
  string port encoding.

* test-suite/tests/print.test (tprint): Use 'set-port-encoding!' to set
  the string port encoding.
  ("truncated-print"): Use 'pass-if-equal'.

* test-suite/tests/ports.test ("encoding failure leads to exception",
  "%default-port-encoding is honored", "peek-char [latin-1]", "peek-char
  [utf-8]", "peek-char [utf-16]"): Remove tests.
  ("%default-port-encoding is ignored", "peek-char"): Add tests.
  ("suitable encoding [latin-1]", "suitable encoding [latin-3]",
  "wrong encoding, error", "wrong encoding, substitute",
  "wrong encoding, escape"): Use 'set-port-encoding!' to set the
  string port encoding.
  ("%default-port-encoding, wrong encoding"): Rewrite to use
  a file port instead of a string port.

doc/ref/srfi-modules.texi

@@ -146,6 +146,7 @@ guile-2  ;; starting from Guile 2.x
 r5rs
 srfi-0
 srfi-4
+srfi-6
 srfi-13
 srfi-14
 srfi-23
@@ -1851,19 +1852,11 @@ uniform numeric vector, it is returned unchanged.
 @cindex SRFI-6
 
 SRFI-6 defines the procedures @code{open-input-string},
-@code{open-output-string} and @code{get-output-string}.
-
-Note that although versions of these procedures are included in the
-Guile core, the core versions are not fully conformant with SRFI-6:
-attempts to read or write characters that are not supported by the
-current @code{%default-port-encoding} will fail.
-
-We therefore recommend that you import this module, which supports all
-characters:
-
-@example
-(use-modules (srfi srfi-6))
-@end example
+@code{open-output-string} and @code{get-output-string}.  These
+procedures are included in the Guile core, so using this module does not
+make any difference at the moment.  But it is possible that support for
+SRFI-6 will be factored out of the core library in the future, so using
+this module does not hurt, after all.
 
 @node SRFI-8
 @subsection SRFI-8 - receive