[krbdev.mit.edu #7937] pkinit_identities should support path substitution

[Greg Hudson via RT]

On a multi-user machine, it is not convenient to set up PKINIT so that 
client certificates are obtained from each user's home directory.  At 
best, you can specify pkinit_identities = ENV:envvarname and put an 
environment variable setting in every user's dotfiles.

In 1.11 we introduced a path substitution facility borrowed from Heimdal, 
which could be applied to this purpose, especially if we added a %{home} 
token for the home directory.

Here is an example of an administrator wanting to use path substitution 
for pkinit_identities:

http://mailman.mit.edu/pipermail/kerberos/2014-June/019922.html