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.

It now filters out the remaining lines that are issued by make. Also
print diagnistics only once per failed test program.

The new implementation gives much better diagnistics in case something
goes wrong, including the whole compiler (snc) output. It is also
simpler and uses Test::More in a more idiomatic way.