add net support

os/src/syscall/net.rs

@@ -0,0 +1,14 @@
+use alloc::sync::Arc;
+use crate::net::IPv4;
+use crate::net::udp::UDP;
+use crate::task::current_process;
+
+// just support udp
+pub fn sys_connect(raddr: u32, lport: u16, rport: u16) -> isize {
+    let process = current_process();
+    let mut inner = process.inner_exclusive_access();
+    let fd = inner.alloc_fd();
+    let udp_node = UDP::new(IPv4::from_u32(raddr), lport, rport);
+    inner.fd_table[fd] = Some(Arc::new(udp_node));
+    fd as isize    
+}