On the log-addressing branch:  Follow-up to r1511324.

Address various compatibility check issues.  First, the compat
version precedence check has to be reversed.  Second, we should
select the oldest version of these:

- current tool version (takes care of wrong major versions as well)
- SVN_FS_CONFIG_COMPATIBLE_VERSION
- SVN_FS_CONFIG_PRE_1_*_COMPATIBLE

Also, we want the patch level to have a defined value.  We set
it to 0 as there must be no format change in a patch release.

* subversion/include/private/svn_fs_util.h
  (svn_fs__compatible_version): update docstring

* subversion/libsvn_fs_util/fs-util.c
  (add_compatility): new utility to combine version requirements
  (svn_fs__compatible_version): check versions in proper order;
                                limit result to current version;
                                set patch level to 0

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-addressing@1511346 13f79535-47bb-0310-9956-ffa450edef68