Avoid compiler warnings in util/network.c with old systems

If the system doesn't support IP_FREEBIND, ensure we don't create
an if statement with an empty body to avoid compiler warnings.
Reported by Julien ÉLIE with Debian etch and gcc 4.1.2.
Avoid compiler warnings on pragmas with pre-4.2 gcc

When disabling -Wformat-nonliteral in the snprintf test suite,
only use the pragma for GCC 4.2 and later.  Older versions don't
support that warning flag and will warn about the pragma.
Use INVALID_SOCKET instead of -1 in util/network.h comments
Add UNUSED annotation to two internal network functions

If a system doesn't support IPv6, but does support some of the
IPv6 constants (such as when IPv6 is disabled for some reason),
this would produce warnings about functions that are defined but
not used.  This should be quite rare, but add UNUSED annotations
to the functions anyway, since there should be no harm.
Conditionalize include of sys/time.h and add time.h for Windows

Windows doesn't have sys/time.h and defines struct timeval in
time.h.  Conditionalize the inclusion of sys/time.h for Windows
and add an include of time.h where it was missing.

This is mostly pointless, since most of these files won't build
meaningfully on Windows anyway, but may as well be consistent.
Remove unused accept arguments in the network test suite

They weren't cast to the correct type, but since we don't use the
value anyway, no reason to bother.  Just pass in NULL.
Prototype strlcat and strlcpy if not declared

BSD/OS reportedly has implementations of both but doesn't prototype
them in system headers, so switch the prototyping to be conditional
on whether they're declared rather than whether they're available.
Add portable/socket-unix.h portability wrapper

There is a new portable/socket-unix.h portability layer that includes
sys/un.h and defines SUN_LEN if the implementation does not do so.
(Use in combination with the RRA_MACRO_SUN_LEN Autoconf macro.)
Add PIPE_READ and PIPE_WRITE macros

Add PIPE_READ and PIPE_WRITE macros to util/macros.h to name the
elements of the array passed to pipe.