Make the various from_str functions return options

So that they can be used with user input without causing task
failures.

Closes #1335

src/libstd/net.rs

@@ -49,10 +49,13 @@ Failure:
 String must be a valid IPv4 address
 */
 fn parse_addr(ip: str) -> ip_addr {
-    let parts = vec::map(str::split_byte(ip, "."[0]),
-                         {|s| uint::from_str(s) });
+    let parts = vec::map(str::split_byte(ip, "."[0]), {|s|
+        alt uint::from_str(s) {
+          some(n) if n <= 255u { n }
+          _ { fail "Invalid IP Address part." }
+        }
+    });
     if vec::len(parts) != 4u { fail "Too many dots in IP address"; }
-    for i in parts { if i > 255u { fail "Invalid IP Address part."; } }
     ipv4(parts[0] as u8, parts[1] as u8, parts[2] as u8, parts[3] as u8)
 }