Win32: MinGW GCC compatibility.

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.

src/core/ngx_cycle.c

diff --git a/src/core/ngx_cycle.c b/src/core/ngx_cycle.c
index 2c006bc..eb39ab2 100644
--- a/src/core/ngx_cycle.c
+++ b/src/core/ngx_cycle.c
@@ -543,7 +543,7 @@
                 }
             }
 
-            if (nls[n].fd == -1) {
+            if (nls[n].fd == (ngx_socket_t) -1) {
                 nls[n].open = 1;
             }
         }
@@ -649,7 +649,7 @@
     ls = old_cycle->listening.elts;
     for (i = 0; i < old_cycle->listening.nelts; i++) {
 
-        if (ls[i].remain || ls[i].fd == -1) {
+        if (ls[i].remain || ls[i].fd == (ngx_socket_t) -1) {
             continue;
         }
 
@@ -813,7 +813,7 @@
 
     ls = cycle->listening.elts;
     for (i = 0; i < cycle->listening.nelts; i++) {
-        if (ls[i].fd == -1 || !ls[i].open) {
+        if (ls[i].fd == (ngx_socket_t) -1 || !ls[i].open) {
             continue;
         }