Make "svnadmin create --fs-type=bdb" warn about the BDB back-end deprecation.

* subversion/svnadmin/svnadmin.c (subcommand_create): Print a warning to stderr
   if the requested filesystem type is "bdb".

* subversion/tests/cmdline/svntest/main.py (create_repos): Expect the output
   of "svnadmin create" to contain that warning.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1464985 13f79535-47bb-0310-9956-ffa450edef68