fixing dialogCommand and printing.pnum. Refs: #48.
Ad proposed by Josef Templ in CPC 1.7 rc5.
- printing.pnum is incremented globally, i.e. at the outermost print job level.
- dialogCommand now uses the standard form for opening a resource file, which avoids
nasty side effects that occur with the previously used and undocumented form. It used to open the resource file without the proper capitalization of the file name and thereby confused case sensitive tools such as a Novell file server, git and svn.

Signed-off-by: Josef Templ <josef.templ@gmail.com>