Polish the syslog messages to contain readily useful information.
Behavior of capability 'priority' is inconsistent with what is done for all
other contexts: 'umask', 'cpumask', resource limits, etc., where an absence of
capability means to inherit the value. It is currently preserved for
compatibility.
Remove LOGIN_DEFPRI since it is unused in-tree, GitHub's code search and Google
show no use, and it doesn't exist in OpenBSD and NetBSD.
PR: 271749