select, poll, xpoll: fix compilation warnings

Like this:

-----
network/select.c:123:60: warning: pointer targets in passing argument 5 of 'getsockopt' differ in signedness [-Wpointer-sign]
  123 |              getsockopt( fd, SOL_SOCKET, SO_TYPE, &optval, &optlen ) == 0 )
      |                                                            ^~~~~~~
      |                                                            |
      |                                                            int *
-----

network/poll.c

@@ -37,13 +37,16 @@
 #include <sys/socket.h>
 #include <sys/stat.h>
 
+#include "socklen_t.h"
+
 #include "poll.h"
 
 static int checkfd( int fd )
 {
     struct stat st;
     ULONG ulState;
-    int optval, optlen = sizeof( optval );
+    int optval;
+    socklen_t optlen = sizeof( optval );
 
     /* invalid handle */
     if( fstat( fd, &st ) == -1 )

network/select.c

@@ -29,6 +29,8 @@
 #include <sys/select.h>
 #include <sys/stat.h>
 
+#include "socklen_t.h"
+
 /* alias */
 int _std_select( int, fd_set *, fd_set *, fd_set *, struct timeval * );
 
@@ -97,7 +99,8 @@ static int setfd( int fd, PSELECTPARM parms, int op )
     ULONG ulType, ulAttr;
 #endif
     ULONG ulState;
-    int optval, optlen = sizeof( optval );
+    int optval;
+    socklen_t optlen = sizeof( optval );
     int type;
 
     if( fstat( fd, &st ) == -1 )

network/xpoll.c

@@ -25,6 +25,8 @@
 #include <sys/stat.h>
 #include <sys/socket.h>
 
+#include "socklen_t.h"
+
 #include "xpoll.h"
 
 struct os2compat_xpollset
@@ -103,7 +105,7 @@ static int check_fd( int fd )
     struct stat st;
     ULONG ulState;
     int optval;
-    int optlen = sizeof( optval );
+    socklen_t optlen = sizeof( optval );
 
     /* accept negative fd, but do nothing for it like poll() */
     if( fd < 0 )