modem-helpers: improve and fix COPS=? numeric fields parsing

The numeric fields in the +COPS=? response were relying on a very weak
parsing logic, assuming that they were single-digit numeric values and
not using the common string to integer conversion utilities.

This commit improves the conversion from the 3GPP/ETSI defined network
availability and access technology values to the MM defined ones,
providing enum-based matches even if the numeric values are the same.

The commit also fixes the parsing of access technology values > 10,
required to report 5G related values.

src/tests/test-modem-helpers.c

@@ -961,6 +961,7 @@ test_cops_query_data (const CopsQueryData *item)
                                                &format,
                                                &operator,
                                                &act,
+                                               NULL,
                                                &error);
     g_assert_no_error (error);
     g_assert (result);