globals: Add function saveIfFile()

This is an alternative function to globals' saveFile(), which allows
model code to just pass in the name of the expected uploaded file,
rather than requiring them to look up the file themselves.  This is in
line with my preference to encapsulate PHP superglobals access away from
most of the codebase.

Note that even if the user opts not to upload optional files, the
associated file <input> field will still be present in $_FILES, with a
special error code set (meaning 'no file uploaded') which setFile()
ignores.  It is only in the case of a malformed form submission that
$_FILES will be missing the requested file field, prompting Scrott to
throw an exception.

0 files changed, 0 insertions, 0 deletions