refactor: use string prefix instead of function in shell matchers.

* lib/specl/shell.lua (ProcessMatcher._init)
(ProcessMatcher._parmtypes): ProcessMatcher constructor takes
just two unnamed method arguments.
(ProcessMatcher:format_expect)
(ProcessMatcher:format_alternatives): Set default methods to
automatically construct resport strings from expecting field.
Adjust and simplify all instantiations accordingly.

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