Detect PDF files without .pdf extension using magic number

This patch checks whether input files that do not have a .pdf extension have "%PDF" as their first four bytes. If the file has this magic number, it treats them as a PDF. The PDF standard requires that PDFs begin with that magic number, per [Adobe's PDF spec](http://wwwimages.adobe.com/www.adobe.com/content/dam/Adobe/en/devnet/pdf/pdfs/adobe_supplement_iso32000.pdf). 

Acrobat can non-standard open files that have the magic number anywhere in the first 1024 bytes (see page 108). This patch won't detect those non-standard files: I think the extra computational cost isn't worth the effort for an ever-smaller set of non-standard PDFs without a .pdf extension.

If someone had a plaintext file that began with %PDF, this patch would have docsplit treat it as a PDF. I don't think this is a serious problem: for instance, Ubuntu treats anything beginning with "%PDF-" as a PDF if it lacks a file extension.