The new function protocol_supports_flag() checks the protocol flags
to verify if some features are supported, but will support being
extended to refine the tests. Let's use it to check for REUSEPORT.

We'll use these flags to know if some protocols are supported, and if
so, with what options/extensions. Reuseport will move there for example.
Two functions were added to globally set/clear a flag.

At a few places we were still using protocol_by_family() instead of
the richer protocol_lookup(). The former is limited as it enforces
SOCK_STREAM and a stream protocol at the control layer. At least with
protocol_lookup() we don't have this limitationn. The values were still
set for now but later we can imagine making them configurable on the
fly.

Instead of using sock_type and ctrl_type to select a protocol, let's
make use of the new protocol type. For now they always match so there
is no change. This is applied to address parsing and to socket retrieval
from older processes.

Commit ab0a5192a ("MEDIUM: config: mark "grace" as deprecated") marked
the "grace" keyword as deprecated in 2.3, tentative removal for 2.4
with a hard deadline in 2.5, so let's remove it and return an error now.
This old and outdated feature was incompatible with soft-stop, reload
and socket transfers, and keeping it forced ugly hacks in the lower
layers of the protocol stack.

This will instantly stop all listeners except those which belong to
a proxy configured with a grace time. This means that UDP listeners,
and peers will also be stopped when called this way.

These two functions are used to pause and resume all listeners of
all protocols. They use the standard listener functions for this
so they're supposed to handle the situation gracefully regardless
of the upper proxies' states, and they will report completion on
proxies once the switch is performed.

It might be nice to define a particular "failed" state for listeners
that cannot resume and to count them on proxies in order to mention
that they're definitely stuck. On the other hand, the current
situation is retryable which is quite appreciable as well.

The protocol array used to be only indexed by socket family, which is very
problematic with UDP (requiring an extra family) and with the forthcoming
QUIC (also requiring an extra family), especially since that binds them to
certain families, prevents them from supporting dgram UNIX sockets etc.

In order to address this, we now start to register the protocols with more
info, namely the socket type and the control type (either stream or dgram).
This is sufficient for the protocols we have to deal with, but could also
be extended further if multiple protocol variants were needed. But as is,
it still fits nicely in an array, which is convenient for lookups that are
instant.

Ever since the protocols were added in 1.3.13, listeners used to be
started twice:
  - once by start_proxies(), which iteratees over all proxies then all
    listeners ;
  - once by protocol_bind_all() which iterates over all protocols then
    all listeners ;

It's a real mess because error reporting is not even consistent, and
more importantly now that some protocols do not appear in regular
proxies (peers, logs), there is no way to retry their binding should
it fail on the last step.

What this patch does is to make sure that listeners are exclusively
started by protocols. The failure to start a listener now causes the
emission of an error indicating the proxy's name (as it used to be
the case per proxy), and retryable failures are silently ignored
during all but last attempts.

The start_proxies() function was kept solely for setting the proxy's
state to READY and emitting the "Proxy started" message and log that
some have likely got used to seeking in their logs.

The protocol.h files are pretty low in the dependency and (sadly) used
by some files from common/. Almost nothing was changed except lifting a
few comments.