Change `sendfile' to loop until everything has been sent.

Change `sendfile' to loop until everything has been sent.

* libguile/filesys.c (scm_sendfile)[HAVE_SYS_SENDFILE_H &&
  HAVE_SENDFILE]: Compare RESULT with C_COUNT.  Loop until C_COUNT bytes
  have been sent.
* doc/ref/posix.texi (File System): Update the description.  Explain the
  new semantics.
* test-suite/tests/filesys.test ("sendfile"): Rewrite using
  `pass-if-equal'.  Check the return value for all the tests.
  ["file with offset past the end", "file with offset near the end"]:
  New tests.