CLEANUP: config: set the maxaccept value for peers listeners earlier

Since we introduced bind_conf in peers, we can set maxaccept in a cleaner
way at the proper time, let's do this to make the code more readable.
BUG/MINOR: backend: only match IPv4 addresses with RDP cookies

The RDP cookie extractor compares the 32-bit address from the request
to the address of each server in the farm without first checking that
the server's address is IPv4. This is a leftover from the IPv4 to IPv6
conversion. It's harmless as it's unlikely that IPv4 and IPv6 servers
will be mixed in an RDP farm, but better fix it.

This patch does not need to be backported.
BUG/MINOR: checks: correctly configure the address family and protocol

Currently, mixing an IPv4 and an IPv6 address in checks happens to
work by pure luck because the two protocols use the same functions
at the socket level and both use IPPROTO_TCP. However, they're
definitely wrong as the protocol for the check address is retrieved
from the server's address.

Now the protocol assigned to the connection is the same as the one
the address in use belongs to (eg: the server's address or the
explicit check address).
MINOR: tools: split is_addr() and is_inet_addr()

The is_addr() function indicates if an address is set and is an IPv4
or IPv6 address. Let's rename it is_inet_addr() and make is_addr()
also accept AF_UNIX addresses.
MINOR: protocols: use is_inet_addr() when only INET addresses are desired

We used to have is_addr() in place to validate sometimes the existence
of an address, sometimes a valid IPv4 or IPv6 address. Replace them
carefully so that is_inet_addr() is used wherever we can only use an
IPv4/IPv6 address.
MEDIUM: unix: add preliminary support for connecting to servers over UNIX sockets

We've had everything in place for this for a while now, we just missed
the connect function for UNIX sockets. Note that in order to connect to
a UNIX socket inside a chroot, the path will have to be relative to the
chroot.

UNIX sockets connect about twice as fast as TCP sockets (or consume
about half of the CPU at the same rate). This is interesting for
internal communications between SSL processes and HTTP processes
for example, or simply to avoid allocating source ports on the
loopback.

The tcp_connect_probe() function is still used to probe a dataless
connection, but it is compatible so that's not an issue for now.

Health checks are not yet fully supported since they require a port.
MEDIUM: checks: only complain about the missing port when the check uses TCP

For UNIX socket addresses, we don't need any port, so let's disable the
check under this condition.
MEDIUM: unix: implement support for Linux abstract namespace sockets

These sockets are the same as Unix sockets except that there's no need
for any filesystem access. The address may be whatever string both sides
agree upon. This can be really convenient for inter-process communications
as well as for chaining backends to frontends.

These addresses are forced by prepending their address with "abns@" for
"abstract namespace".