For issue #3913, in case an authentication failure occurred and the '-F -'
option was given, make 'svn' point out that '-F -' can interfere with
interactive prompting.  

* subversion/svn/svn.c
  (main): Keep track of whether -F is causing us to read from standard input,
   and, if so, append an appropriate hint to authentication error messages.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1467597 13f79535-47bb-0310-9956-ffa450edef68