Make subprocess functions resolve the default directory

Make subprocess functions resolve the default directory

`call-process' doesn't respect file name handlers in
`default-directory', so `file-name-non-special' has to resolve them
for `process-file', `start-file-process', and
`shell-command' (Bug#25949).

* lisp/files.el (file-name-non-special): Also resolve default
directory for 'process-file', 'start-file-process', and
'shell-command'.
* test/lisp/files-tests.el
(files-tests--file-name-non-special--subprocess): Add unit test.