add several functions, distinguish canonical from resolve, bugfixes

add function fileio.{parent,stem,filename,suffix,join}

distinguish function fileio.canonical() which returns relative path if not exist from fileio.resolve() that always returns absolute path--if not exist, absolute based on cwd.

expanduser: ignore inputs like ~abc

subprocess_run: correct high level interface nargout and improve test

absolute("") = ""

Internals use posix filesep "/" throughout

Test: shortname: test with Matlab exe so it's always present (vs. notepad)