The new boolean member multi_pv of struct func_symbol represents whether
multi-pv arguments are allowed by default, i.e with -p in effect. The code
generator takes this into account when deciding what to do for a multi-PV
argument. The behaviour is now exactly as described in the release notes.

This tests rejection and warnings about bad applications of type operators.

This (large) patch also makes related changes to a number of identifiers.

We now build normal syntax nodes during parsing, re-using node types from
the expression syntax (E_VAR, E_BINOP, E_PRE, E_SUBSCR, E_FUNC). Together
with the recent change in generating types, this means we no longer need the
backward parent links in the type structure. Most of the code in var_types.c
is now much easier to understand and modify.

Also made the tentative change to represent "const" not as a type node with
a child pointer, but rather with a boolean flag. This is in anticipation of
adding the "pv" type operator in the hope that using yet another flag to
represent it will make analysis of a type easier. It could well be that I
revert this change if it turns out that this makes things more difficult. A
first indication is that in var_types.c this already complicates some of the
routines. So I left most of the T_CONST cases in there, only disabled them
with #if 0...#endif.

We currently use it only to find out whether a given expression has
function type (or pointer to function), because in that case we must
supply arguments for the implicit parameters.