plugin-base: new 'allowed-capabilities' property

The plugins can set this property to filter support check requests by probed
capabilities.
The value given to the property should be a guint built as a mask of
MM_PORT_PROBE_CAPABILITY flags, e.g.:

    const guint capabilities = (MM_PORT_PROBE_CAPABILITY_GSM |
                                MM_PORT_PROBE_CAPABILITY_CDMA);

src/mm-plugin-base.h

@@ -124,6 +124,7 @@ typedef struct {
 } MMPluginCustomInit;
 
 #define MM_PLUGIN_BASE_NAME                    "name"
+#define MM_PLUGIN_BASE_ALLOWED_CAPABILITIES    "allowed-capabilities"
 #define MM_PLUGIN_BASE_ALLOWED_SUBSYSTEMS      "allowed-subsystems"
 #define MM_PLUGIN_BASE_ALLOWED_VENDOR_IDS      "allowed-vendor-ids"
 #define MM_PLUGIN_BASE_ALLOWED_PRODUCT_IDS     "allowed-product-ids"