* Get rid of the concept of `modspec': `getarg' now has `require' for
require specs and `module' for a module name (the latter is what all
previous uses of 'modspec except for ,require really needed); command
descriptions use "<require-spec>" and "<module>", documentation
adjusted as well.
* `module-name?' etc turn to `known-module' and `known-module-name',
with a saner behavior, and tests to keep it sane.
* This cleans up a lot of things. Two specific points: ,switch works
better with toplevel-defined modules (see the corresponding change in
the test suite), and also fixes PR 12148.
* Ensure that ,sh commands return void.
* Add tests for ,r with non-atomic require spec, and for use of $F in
,sh commands.
* Improved the test suite, including uses of `module+' so each file can
be run by itself to perform a subset of the tests.
