MINOR: set IP_FREEBIND on IPv6 sockets in transparent mode

Lets set IP_FREEBIND on IPv6 sockets as well, this works since Linux 3.3
and doesn't require CAP_NET_ADMIN privileges (IPV6_TRANSPARENT does).

This allows unprivileged users to bind to non-local IPv6 addresses, which
can be useful when setting up the listening sockets or when connecting
to backend servers with a specific, non-local source IPv6 address (at that
point we usually dropped root privileges already).
BUG/MINOR: raw_sock: also consider ENOTCONN in addition to EAGAIN for recv()

I was testing haproxy-1.5-dev22 on SmartOS (an illumos-based system)
and ran into a problem.  There's a small window after non-blocking
connect() is called, but before the TCP connection is established,
where recv() may return ENOTCONN.  On Linux, the behaviour here seems
to be always to return EAGAIN.  The fix is relatively trivial, and
appears to make haproxy work reliably on current SmartOS (see patch
below).  It's possible that other UNIX platforms exhibit this
behaviour as well.

Note: the equivalent was already done for send() in commit 0ea0cf6
("BUG: raw_sock: also consider ENOTCONN in addition to EAGAIN").
Both patches should be backported to 1.4.