fix 396851: move absolutizing relative path (added by Ted) out of sp_export_png_file...

fix 396851: move absolutizing relative path (added by Ted) out of sp_export_png_file and make sure it works only for rel filenames from hints (both on command line and on batch export), but not for filenames specified on command line or in export dialog; reenable export-id with export-use-hints; fix errors in string comparisons when choosing command line mode; fix crash when opening nonexistent file in command line mode