Tweak the initial pool in Perl bindings, so that perl -MSVN::Core -c -e1 won't segfault.
* perl/native/Core.pm: (gpool): New function for returning and holding the global pool. This is to avoid 'our's which appears to have different destruction order. (SVN::Pool::DESTROY): Skip destroy for SCALAR-typed SVN::Pool, as they appear to be caused by bugs in Perl.
* perl/native/Ra.pm: Use the gpool method to get the pool for init_ra_libs.