Handle Proxy Protocol v2 safely as well.

Handle Proxy Protocol v2 safely as well.

We had test suite failures (test suite success!) because Proxy Protocol
v2 (PPv2) wasn't being detected; by only reading 12 octets, the >= 16
check was failing.  But in fact I had previously only fixed reading
"only enough" for PPv1.

Handling both PPv1 and PPv2 is complicated because the minimum valid
length for PPv1 is 15 octets but for PPv2 the size to read is in the
15th and 16th octets.

So refactored a little and we now use a total of 3 reads for the PPv2
case (assuming no fragmentation, etc; we'll actually keep reading now
instead of aborting) to get the entire PPv2 header of exactly the right
size, so that TLS handshake immediately following the PP header is not
also swallowed.

Fixes: 2018
Tested: manually, TLS and non-TLS, PPv1 and PPv2, all ways.
Release: should be cherry-picked into 4.89RC series