Informative envvars for hooks

[Daniel Shahaf <da...@elego.de>]

We could set some informative variables in the hooks' environment, such as:

- $SVN_MAJOR_VERSION, $SVN_MINOR_VERSION, $SVN_PATCH_VERSION
  Same as the #define's of the same name.

- $SVN_SERVER_CAPABILITIES 
  Example: by advertising here that the server has the "validate
  revprops are in LF" capability, hooks would know to skip doing
  those validations themselves when run under a new server.

- $SVN_CLIENT_IP
  So people don't have to manually compute it by tailing their log files.

I'm sure y'all have other ideas :).  Are there particular bits of
information we'd like to make available this way?