zlisp: make 't => true and 'nil => false mapping explicit.

* lib/zemacs/eval.lua (marshaller): Insert false into Lua
argument list when evaluation returns 'nil.
Return Lua subr call result unmolested.
(call_command, eval_string, execute_function): Name the result
holding variable `value` rather than `ok` so as not to hint that
it is a boolean.
* lib/zemacs/zlisp/minibuf.zl (message): Convert Lua internal
values (false, true, Symbol table) back to Zlisp write syntax
representation ('nil, 't, symbol name) before display.
* lib/zemacs/specs/zlisp_lisp_spec.yaml (progn): Remove pending
call from newly passing examples.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>