Restrict the FSFS L2P index page size to powers of two.  That's easier
to verify and consistent with all the other settings we have.

* subversion/libsvn_fs_fs/fs_fs.c
  (write_config): Update and clean up fsfs.conf docstring.

* subversion/libsvn_fs_fs/index.c
  (get_l2p_header_body): Verify that the L2P page size is 2^N.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1622963 13f79535-47bb-0310-9956-ffa450edef68