config: add ‘none’ as a possible value for ‘selection-target’

When ‘selection-target’ is set to ‘none’, selecting text does not copy
the text to _any_ clipboard.

This patch also refactors the value parsing to be data driven.

selection.c

@@ -1019,6 +1019,9 @@ selection_finalize(struct seat *seat, struct terminal *term, uint32_t serial)
     xassert(term->selection.start.row <= term->selection.end.row);
 
     switch (term->conf->selection_target) {
+    case SELECTION_TARGET_NONE:
+        break;
+
     case SELECTION_TARGET_PRIMARY:
         selection_to_primary(seat, term, serial);
         break;