BUILD: checks: external checker needs signal.h

check.c doesn't build on solaris since 98637e5 ("MEDIUM: Add external check")
because sigset_t is unknown. Simply include signal.h to fix the issue.

No need to backport, this is 1.6-only.
BUILD: checks: kill a minor warning on Solaris in external checks

Gcc on Solaris complains that elem->pid is pid_t and that we display
it as int. A simple cast fixes this. No backport needed, this is 1.6
only.
BUILD: http: fix isdigit & isspace warnings on Solaris

As usual, when touching any is* function, Solaris complains about the
type of the element being checked. Better backport this to 1.5 since
nobody knows what the emitted code looks like since macros are used
instead of functions.
BUG/MINOR: listener: set the listener's fd to -1 after deletion

This is currently harmless, but when stopping a listener, its fd is
closed but not set to -1, so it is not possible to re-open it again.
Currently this has no impact but can have after the abstract sockets
are modified to perform a complete close on soft-reload.

The fix can be backported to 1.5 and may even apply to 1.4 (protocols.c).
BUG/MEDIUM: unix: failed abstract socket binding is retryable

Jan Seda noticed that abstract sockets are incompatible with soft reload,
because the new process cannot bind and immediately fails. This patch marks
the binding as retryable and not fatal so that the new process can try to
bind again after sending a signal to the old process.

Note that this fix is not enough to completely solve the problem, but it
is necessary. This patch should be backported to 1.5.
MEDIUM: listener: implement a per-protocol pause() function

In order to fix the abstact socket pause mechanism during soft restarts,
we'll need to proceed differently depending on the socket protocol. The
pause_listener() function already supports some protocol-specific handling
for the TCP case.

This commit makes this cleaner by adding a new ->pause() function to the
protocol struct, which, if defined, may be used to pause a listener of a
given protocol.

For now, only TCP has been adapted, with the specific code moved from
pause_listener() to tcp_pause_listener().
MEDIUM: listener: support rebinding during resume()

When a listener resumes operations, supporting a full rebind makes it
possible to perform a full stop as a pause(). This will be used for
pausing abstract namespace unix sockets.
BUG/MEDIUM: unix: completely unbind abstract sockets during a pause()

Abstract namespace sockets ignore the shutdown() call and do not make
it possible to temporarily stop listening. The issue it causes is that
during a soft reload, the new process cannot bind, complaining that the
address is already in use.

This change registers a new pause() function for unix sockets and
completely unbinds the abstract ones since it's possible to rebind
them later. It requires the two previous patches as well as preceeding
fixes.

This fix should be backported into 1.5 since the issue apperas there.
DOC: explicitly mention the limits of abstract namespace sockets

Listening to an abstract namespace socket is quite convenient but
comes with some drawbacks that must be clearly understood when the
socket is being listened to by multiple processes. The trouble is
that the socket cannot be rebound if a new process attempts a soft
restart and fails, so only one of the initially bound processes
will still be bound to it, the other ones will fail to rebind. For
most situations it's not an issue but it needs to be indicated.