With this change it is now possible to load modules compiled without
the "--with-http_ssl_module" configure option into nginx binary compiled
with it, and vice versa (if a module doesn't use ssl-specific functions),
assuming both use the "--with-compat" option.

With this change it is now possible to load modules compiled without
the "--with-file-aio" configure option into nginx binary compiled with it,
and vice versa, assuming both use the "--with-compat" option.

With this change it is now possible to load modules compiled without
the "--with-threads" configure option into nginx binary compiled with it,
and vice versa (if a module does not use thread-specific functions),
assuming both use the "--with-compat" option.

Linux can return AF_UNIX sockaddrs with partially filled sun_path,
resulting in spurious comparison failures and failed binary upgrades.
Added proper checking of the lengths provided.

Reported by Jan Seda,
http://mailman.nginx.org/pipermail/nginx-devel/2016-September/008832.html.

Previously, user access bits were always set to "rw" unconditionally,
even with "user:r" explicitly specified.  With this change we only add
default user access bits (0600) if they weren't set explicitly.

The new parameters "manager_files", "manager_sleep"
and "manager_threshold" were added to proxy_cache_path
and friends.

Note that ngx_path_manager_pt was changed to return ngx_msec_t
instead of time_t (API change).

c->pool is not destroyed here since c52408583801.

Removed (NGX_HAVE_DEFERRED_ACCEPT && defined TCP_DEFER_ACCEPT)
from the signature accordingly.

Removed NGX_HAVE_REUSEPORT from the signature accordingly.

The IPV6_V6ONLY macro is now checked only while parsing appropriate flag
and when using the macro.

The ipv6only field in listen structures is always initialized to 1,
even if not supported on a given platform.  This is expected to prevent
a module compiled without IPV6_V6ONLY from accidentally creating dual
sockets if loaded into main binary with proper IPV6_V6ONLY support.