On the fsfs-format7 branch: make 'svnadmin create --compatible-version"
accept 1.9.

* subversion/include/svn_fs.h
  (SVN_FS_CONFIG_PRE_1_9_COMPATIBLE): new internal option
* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__create): handle new option
* subversion/svnadmin/main.c
  (subcommand_create): convert "compatible version" into internal option


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1432696 13f79535-47bb-0310-9956-ffa450edef68