svnauthz: Grammar and Docstring Police.

* tools/server-side/svnauthz.c
  (svnauthz__is, subcommand_accessof, subcommand_help, sub_main):
    Improve usage and error messages and comments.
  (SVNAUTHZ_COMPAT_NAME, use_compat_mode): Add docstrings.
    Also compute string length at compile time.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1428651 13f79535-47bb-0310-9956-ffa450edef68