In theory, this can provide a bit better distribution of latencies.
Also it simplifies the code, since ngx_queue_t is now used instead
of custom implementation.
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.
