@(collect): don't default vars if all required missing.

The @(collect) directive disallows the situation when
there are required vars, but some are missing (not bound
by the collect body). However, the special case is allowed
when none of the required variables are bound; that
doesn't trigger the exception. There is a poor specification
in this area: the issue is that when there are optional
variables, and all variables are missing (optional and
required), the optional ones are still bound to their
default values.  Thus, the situations is half-baked: some of
the :vars are bound and some are not. This violates the
all-or-nothing principle of :vars.

This patch addresses the poor specification: if all variables
are missing, then the optional variables are not bound to
their defaults.

* match.c (h_collect, h_coll): Detect the situation when at
least one variable is required, and all optional variables are
defaulted. In this case, don't propagate any bindings to the
collected lists.

* txr.1: Doc updated.

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