zlc: isolate namespaces with 'do ... end' blocks.

When compiling several zlisp files, we don't want locals from one
compiled file to be visible in another compiled file output to
the same object.
* lib/zemacs/zlc: Wrap the results of each compiled file in a
'do ... end' block, and add a header comment naming the original
source it came from.

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