Fixed issue #2147 - control characters in log message cause failure (over DAV)

* subversion/mod_dav_svn/log.c: modified log_receiver to call
    svn_xml_fuzzy_escape()

* subversion/include/svn_xml.h,
  subversion/libsvn_subr/xml.c: added svn_xml_fuzzy_escape()

* subversion/tests/clients/cmdline/log_tests.py:
    added escape_control_chars test