ps-to-png.scm: only use -dEPSCrop for EPS files.

ps-to-png.scm: only use -dEPSCrop for EPS files.

The previous check for multi-page documents fails to account for
single-page PS files, which must have page sizes specified using
-dDEVICEWIDTHPOINTS and -dDEVICEHEIGHTPOINTS when converted to PNG.

This commit improves the situation: only use the -dEPSCrop option for
EPS files, which do not have a page count.  All files that specify a
page count (PS files), will now be converted to PNGs with correct
dimensions.