If a PAM module wants to authenticate user using GSSAPI, the authentication
is broken if non-default ccache name is used in KRB5CCNAME environment
variable.
One way to mitigate this would be to add this to env_keep, but this also
makes the variable available in the executed command which may not
be always desirable.
This patch sets KRB5CCNAME for pam_authenticate only, if it is available
and not yet set.
3b7977a42c