Provide the same logging detail on stderr in "run once" mode that
we otherwise have in svnserve's log file.

* subversion/svnserve/logger.h
  (logger__create_for_stderr): declare new logger constructor

* subversion/svnserve/logger.c
  (logger_t): write to streams instead of files
  (logger__create_for_stderr): implement new constructor
  (logger__create,
   logger__log_error,
   logger__write): update existing constructor to streams

* subversion/svnserve/svnserve.c
  (sub_main): log to stderr when "run once" mode

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544731 13f79535-47bb-0310-9956-ffa450edef68