Fix svnserve logging in multi-threaded servers and make them log all errors.

We simply use a mutex alongside the log file to serialize all access to it.
That also allows us to use the log file in our worker thread funtion.

* subversion/svnserve/server.h
  (server_baton_t,
   serve_params_t): add mutexes next to the log file objects
  (log_error): extend signature

* subversion/svnserve/serve.c
  (log_error_internal): factored out from log_error
  (log_error): add serialization support
  (log_server_error,
   serve): update callers

* subversion/svnserve/svnserve.c
  (serve_socket): also log errors
  (main): update caller

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1523502 13f79535-47bb-0310-9956-ffa450edef68