Return a valid error code when adding an invalid IP address.

Currently, passing an invalid IP address to interfaceAddAddress
and friends results in an invalid error message:

    12-02 13:00:42.675 interfaceAddAddress("testtap2355", "2001:db8::1/64", 64) -> ServiceSpecificException(-8, "InterfaceController error: Unknown error -8") <1.63ms>

This is due to confusion between getaddrinfo error codes and
errno values.

Test: new unit test added to IpClientIntegrationTest
Change-Id: Ifdaa4281a9bcf3998e3216472c5c1df0f5285214

libnetutils/ifc_utils.c

@@ -113,6 +113,10 @@ int string_to_ip(const char *string, struct sockaddr_storage *ss) {
     if (ret == 0) {
         memcpy(ss, ai->ai_addr, ai->ai_addrlen);
         freeaddrinfo(ai);
+    } else {
+        // Getaddrinfo has its own error codes. Convert to negative errno.
+        // There, the only thing that can reasonably happen is that the passed-in string is invalid.
+        ret = (ret == EAI_SYSTEM) ? -errno : -EINVAL;
     }
 
     return ret;