Fix various less obvious bugs in PKCS#7 handling: such as not zeroing

the secret key before we've encrypted it and using the right NID for RC2-64.
Add various arguments to the experimental programs 'dec' and 'enc' to make
testing less painful.

This stuff has now been tested against Netscape Messenger and it can encrypt
and decrypt S/MIME messages with RC2 (128, 64 and 40 bit) DES and triple DES.

Its still experimental though...