Add a subversion 1.7 compatibility module

Add a subversion 1.7 compatibility module

While svnrdump is being actively developed as part of Subversion, the
standalone version has been relatively static.  Unfortunately, the
latest code does not work outside the Subversion build environment
because it uses functions that are not part of any released version of
Subversion.

The solution is simple: just provide our own implementation for
those functions.

Introduce a svn17_compat module to do just that, by reusing some
libsvn_subr code from Subversion[1] r981997.  With the exception
of the small modifications needed to adapt it, this code is licensed
from the Apache Software Foundation under the Apache License, which
requires us to include a few other files from the Subversion tree:

 - LICENSE -> LICENSE.subversion
 - NOTICE -> NOTICE.subversion
 - COMMITTERS -> COMMITTERS.subversion

[1] http://svn.apache.org/repos/asf/subversion/trunk

The aforementioned modifications are probably not copyrightable,
but just in case, I hereby release them into the public domain.
You may freely use, modify, distribute, and relicense them.

In theory, the svn17_compat module could shrink over time as simpler
compatibility shims are devised.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>