Experimental streaming PKCS#7 support.

I thought it was about time I dusted this off. This stuff had been sitting on
my hard drive for *ages* (2003 in fact). Hasn't been tested well and may not
work properly.

Nothing uses it at present which is just as well.

Think of this as a traditional Christmas present which looks far more
impressive in the adverts and on the box, some of the bits are missing and
falls to bits if you play with it too much.