Limit the membuffer cache size to half the available address space
typically available (1GB for 32 bit processes).  This prevents running
OOM if the user configured cache sizes too ambitious for their platform.

The code itself is a static, slightly conservative sanity check. It does
not guarantee that the respective platform can actually provide that
much memory nor does it account for platforms that allow for more (e.g.
32 bit app under x64 Windows).

* subversion/libsvn_subr/cache_config.c
  (initialize_cache): Limit the per-process cache size to half of what
                      SVN expects to be addressable.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1645572 13f79535-47bb-0310-9956-ffa450edef68