`ssh-log-pw-blank': known password fields are now omitted from SSH packet logs

`ssh-log-pw-blank': known password fields are now omitted from SSH packet logs
by default (although they can be included). There's also an option to remove
session data, which is good both for privacy and for reducing the size of
logfiles.

git-svn-id: svn://svn.tartarus.org/sgt/putty@4593 cda61777-01e9-0310-a592-d414129be87e