Update port documentation, rename sports to suspendable ports

* module/ice-9/suspendable-ports.scm: Rename from ice-9/sports.scm, and
  adapt module names.  Remove exports that are not related to the
  suspendable ports facility; we want people to continue using the port
  operations from their original locations.  Add put-string to the
  replacement list.
* module/Makefile.am: Adapt to rename.
* test-suite/tests/suspendable-ports.test: Rename from sports.test.
* test-suite/Makefile.am: Adapt to rename.
* module/ice-9/textual-ports.scm (unget-char, unget-string): New
  functions.
* doc/ref/api-io.texi (Textual I/O, Simple Output): Flesh out
  documentation.
  (Line/Delimited): Undocument write-line, read-string, and
  read-string!.  This is handled by (ice-9 textual-ports).
  (Bytevector Ports): Fix duplicated section.
  (String Ports): Move the note about encodings down to the end.
  (Custom Ports): Add explanatory text.  Remove mention of C functions;
  they should use the C port interface.
  (Venerable Port Interfaces): Add text, and make documentation refer to
  recommended interfaces.
  (Using Ports from C): Add documentation.
  (Non-Blocking I/O): Document more fully and adapt to suspendable-ports
  name change.

test-suite/tests/suspendable-ports.test

@@ -17,7 +17,7 @@
 ;;;; <http://www.gnu.org/licenses/>.
 
 (define-module (test-suite test-ports)
-  #:use-module ((ice-9 sports) #:select (install-sports! uninstall-sports!)))
+  #:use-module (ice-9 suspendable-ports))
 
 ;; Include tests from ports.test.
 
@@ -49,10 +49,10 @@
                              #`((include-one #,exp) . #,(lp))))))))
          #:guess-encoding #t)))))
 
-(install-sports!)
+(install-suspendable-ports!)
 
 (include-tests "tests/ports.test")
 (include-tests "tests/rdelim.test")
 (include-tests "tests/r6rs-ports.test")
 
-(uninstall-sports!)
+(uninstall-suspendable-ports!)