Use domain_search_flags in lookup_symbol et al

This changes lookup_symbol and associated APIs to accept
domain_search_flags rather than a domain_enum.

Note that this introduces some new constants to Python and Guile.  I
chose to break out the documentation patch for this, because the
internals here do not change until a later patch, and it seemed
simpler to patch the docs just once, rather than twice.

gdb/printcmd.c

@@ -1582,7 +1582,7 @@ info_address_command (const char *exp, int from_tty)
   if (exp == 0)
     error (_("Argument required."));
 
-  sym = lookup_symbol (exp, get_selected_block (&context_pc), VAR_DOMAIN,
+  sym = lookup_symbol (exp, get_selected_block (&context_pc), SEARCH_VFT,
 		       &is_a_field_of_this).symbol;
   if (sym == NULL)
     {