pam: cache sd-bus separately per module

pam: cache sd-bus separately per module

sd-bus connection is cached by the two pam modules globally, but this
can lead to issues due to hashmaps (used by sd-bus) using a global
static variable for the shared hash key, which is different per module
as both modules are loaded in the same process.

This happens because the sd-bus object is create in one module, but
used in the other, so global state does not match.

Use a different pam cache identifier for the sd-bus pointer, so that
each module uses a different sd-bus connection as a workaround.

Fixes https://github.com/systemd/systemd/issues/27216
Fixes https://github.com/systemd/systemd/issues/17266