[PATCH] Add serf library version information to svn --version

Hi,

I think it will be useful to have svn --version display serf library
version information. Attached patch makes output of svn --version like
this:
[[[
C:\Ivan\SVN\trunk\Debug\subversion\svn>svn.exe --version
svn, version 1.9.0-dev (under development)
   compiled Aug 7 2013, 15:48:17 on x86/x86_64-microsoft-windows6.1.7601

Copyright (C) 2013 The Apache Software Foundation.
This software consists of contributions made by many people;
see the NOTICE file for more information.
Subversion is open source software, see http://subversion.apache.org/

The following repository access (RA) modules are available:

* ra_svn : Module for accessing a repository using the svn network protocol.
  - handles 'svn' scheme
* ra_local : Module for accessing a repository on local disk.
  - handles 'file' scheme
* ra_serf : Module for accessing a repository via WebDAV protocol
using serf 1.2.99.
  - handles 'http' scheme
  - handles 'https' scheme
]]]

It will be very useful for diagnostic serf/ra_serf issues.

Log message:
[[[
Add serf library version to svn --version output.

* subversion/libsvn_ra_serf/serf.c
  (RA_SERF_DESCRIPTION): Add version placeholders.
  (ra_serf_get_description): Add serf library version information to RA layer
   description.
]]]

Thoughts?

-- 
Ivan Zhakov
CTO | VisualSVN | http://www.visualsvn.com