netmgr: Don't crash if socket() returns an error in udpconnect

socket() call can return an error - e.g. EMFILE, so we need to handle
this nicely and not crash.

Additionally wrap the socket() call inside a platform independent helper
function as the Socket data type on Windows is unsigned integer:

> This means, for example, that checking for errors when the socket and
> accept functions return should not be done by comparing the return
> value with –1, or seeing if the value is negative (both common and
> legal approaches in UNIX). Instead, an application should use the
> manifest constant INVALID_SOCKET as defined in the Winsock2.h header
> file.

lib/isc/netmgr/netmgr-int.h

@@ -950,6 +950,12 @@ isc__nm_decstats(isc_nm_t *mgr, isc_statscounter_t counterid);
  * Decrement socket-related statistics counters.
  */
 
+isc_result_t
+isc__nm_socket(int domain, int type, int protocol, uv_os_sock_t *sockp);
+/*%<
+ * Platform independent socket() version
+ */
+
 isc_result_t
 isc__nm_socket_freebind(uv_os_sock_t fd, sa_family_t sa_family);
 /*%<