This allows not modifying priorities for selected login classes (or 'default' on
custom systems).
This also fixes inconsistent behavior compared to what is done for all other
contexts: 'umask', 'cpumask', resource limits, etc., and aligns with what the
login.conf(5) manual page says.
Remove LOGIN_DEFPRI since it is unused in-tree, and GitHub's code search and
Google show no use.
While here, polish the syslog messages to contain readily useful information.
PR: 271749