nixos/pam: add an option to control Kerberos PAM modules
nixos/pam: add an option to control Kerberos PAM modules

Instead of enabling the PAM modules based on config.krb5.enable,
introduce a new option to control the PAM modules specifically.

Users may want to turn on config.krb5.enable, to get a working Kerberos
client config with tools like kinit, while letting pam_sss or something
else handle Kerberos password lookups.

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