Add support for kafs to the -t options of k5start and krenew.  If AFS
setpag support is not compiled in or AFS is not available, but the
Linux kafs kernel module is available, do not error out when -t is
specified and continue to run the external program.  If libkeyutils is
found at compile time and a command was specified on the command line,
create a new session keyring before running the external command to
isolate its AFS credentials from the calling process.  Thanks to Bill
MacAllister, David Howells, and Jeffrey Altman for proposed code and
implementation discussion.