Apply partial loop-unrolling to avoid trace aborts

The JIT compiler has to unroll inner loops in a trace.  It does this up to a (configurable) limit and aborts the trace if it hits that limit.

This commit tries to avoid excessive unroll operations by manually unrolling a few iterations in critical code sections like multi-buffer receive and transmit processing.  This significantly reduces trace aborts in the case of, for example, encapsulation operations that typically use two buffers per packet.  It should not have any negative impact on single-buffer packets.

This could be a middle ground between full-fledged multi-buffer packets and the somewhat radical single-buffer approach proposed by Luke some time ago.

The commit also includes a few deliberate preventions of tailcalls to avoid an effect discussed in the thread
http://www.freelists.org/post/luajit/FFI-methods-for-userdata-objects,3. This is more guesswork than well-founded analysis, though.