Threads and jfifremove

Hi, I've added a couple of features to jpegrescan that you might be interested in.  The best one, IMHO, is threads: try()es can be run in parallel instead of one at a time, using -t.

I also added a Perl version of jfifremove, which removes an 18-byte header that's usually unnecessary.  But since I don't know if every decoder is compatible with that, I put it in a "incompatible" switch with the optimization that was removed earlier for Opera.

Let me know what you think.

Ken