updates to the pkinit code

[Kevin Coffman]

The code in branch users/coffman/pkinit in Subversion
(http://anonsvn.mit.edu/svn/krb5) has been updated.

This code does a better job of splitting the crypto interface and
includes support to use configuration file options and command-line
arguments instead of environment variables.  The file
src/plugins/preauth/pkinit/README has been updated to describe the
configuration changes.

K.C.