This commit introduces a new SECURITY() log macro that logs to facility
'audit' (sometimes called 'security') with severity set to 'alert'. It
is then used to instrument the single most important things to log; all
changes to users and their privileges.
We also add logging to sysrepo, which knows the username for changes to
running-config, copies to startup-config, and RPCs.
Fixes#521
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>