glibc and FreeBSD libc differ in how they handle asprintf allocation
failures.  Make sure we handle both cases properly and clean up the
includes a bit.

Fixes #334

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4436 d4fa192b-c00b-0410-8231-f00ffab90ce4