pam_slurm_adopt: avoid running outside of the sshd PAM service context

This pam module is tailored towards running in the context of remote ssh
logins. When running in a different context like a local sudo call then
the module could be influenced by e.g. passing environment variables
like SLURM_CONF.

By limiting the module to only perform its actions when running in the
sshd context by default this situation can be avoided. An additional pam
module argument service=<service> allows an Administrator to control
this behavior, if different behavior is explicitly desired.

Bug 6184