Fix Proxy Protocol v2 handling

Fix Proxy Protocol v2 handling

Change recv() to not use MSGPEEK and eliminated flush_input().
Add proxy_target_address/port expansions.
Convert ipv6 decoding to memmove().
Use sizeof() for variable sizing.
Correct struct member access.
Enhance debug output when passed invalid command/family.
Add to and enhance documentation.
Client script to test Proxy Protocol, interactive on STDIN/STDOUT,
  so can be chained (ie a swaks pipe), useful for any service, not
  just Exim and/or smtp.