Create a new svn_io_file_flush() wrapper function around the apr variant and
use it to plug some error leaks.

This has the nice side effect to make users make a deliberate decision between
using svn_io_file_flush() and svn_io_file_flush_to_disk() (which more or less
wraps the recently added apr_file_sync())

* subversion/include/svn_io.h
  (svn_io_file_flush): New function.

* subversion/libsvn_ra_serf/commit.c
  (create_put_body): Update caller.

* subversion/libsvn_ra_serf/update.c
  (finish_report): Update caller.

* subversion/libsvn_subr/io.c
  (svn_io_file_flush): New function.

* subversion/libsvn_subr/prompt.c
  (terminal_puts): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483993 13f79535-47bb-0310-9956-ffa450edef68