Introduce a typedef 'svn_fs_freeze_func_t', for source code regularity and
symmetry with 'svn_repos_freeze_func_t'.  It might also be useful for the
SWIG bindings.

* subversion/include/svn_fs.h
  (svn_fs_freeze_func_t): New typedef.
  (svn_fs_freeze): Use it.

* subversion/libsvn_fs_base/fs.c
  (base_bdb_freeze): Use it.

* subversion/libsvn_fs_fs/fs.c
  (fs_freeze): Use it.

* subversion/libsvn_fs/fs-loader.c
  (svn_fs_freeze): Use it.

* subversion/libsvn_fs/fs-loader.h
  (fs_vtable_t): Use it.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1468395 13f79535-47bb-0310-9956-ffa450edef68