Similar to lingering_time, it limits total connection lifetime before
keepalive is switched off.  The default is 1 hour, which is close to
the total maximum connection lifetime possible with default
keepalive_requests and keepalive_timeout.

Also, removed practically unused flag accept_context_updated from
ngx_connection_t.

Several warnings silenced, notably (ngx_socket_t) -1 is now checked
on socket operations instead of -1, as ngx_socket_t is unsigned on win32
and gcc complains on comparison.

With this patch, it's now possible to compile nginx using mingw gcc,
with options we normally compile on win32.

On Linux, sockaddr length is required to process unix socket addresses properly
due to unnamed sockets (which don't have sun_path set at all) and abstract
namespace sockets.