awk macro: implement :begin-file and :end-file.

* share/txr/stdlib/awk.tl (sys:awk-compile-time): New slots,
begin-file-actions and end-file-actions.
(sys:awk-state loop): Take two additional functional arguments
for the begin file and end file actions, and do the
calls in the right places. unwind-protect triggers the
end file function.
(sys:awk-expander): Parse out :begin-file and :end-file
actions.
(awk): Generate lambdas for begin-file and end-file actions,
if they are defined. Pass these to the loop method.
The code is refactored here to do one big sys:awk-let
around everything.

* txr.1: Documented :begin-file and :end-file, revising
the :begin and :end documentation in the process.

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