txr: function span variable must match existing value.

* match.c (h_var_compat): New function; verbatim copy
of existing h_var prior to this commit.
(h_var): If a variable has an existing binding, but
is a function spanning match, do not substitute it with text.
Handle it with the ordinary case, in which we now use
dest_bind instead of cons.
(v_var): Similarly, here, we must also use dest_bind, rather
than always freshly binding the variable.
(match_compat_fixup): For 272 compatibility, substitute
h_var_compat for h_var in the horizontal directive table.

* tests/010/span-var.txr: New test cases.

* txr.1: Documentation updated and also improved overall.
The behavior when a variable has an existing value is
clarified for the regex and fixed field case.
Also update and condense compat notes for 272.

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