refactor: factor format_actual fields into shared functions.

* lib/specl/shell.lua (nonempty_output, but_got_status)
(but_got_status_with_output, but_got_status_with_errout):
Factored out of matchers. Adjust all callers.
(process_errout, reformat_errout): Renamed from these...
(but_got_output, but_got_errout): ...to these, respectively.

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