Make laurel reload its configuration or re-exec via signal

[hillu]

The corrent deployment or update mode relies on auditd stopping/restarting laurel. This has a few shortcomings:

• If laurel has crashed too often, auditd will not restart it even if the binary has changed.
• The workaround, restarting auditd, means that messages will be lost. (This is probably the main reason whssy RHEL systems forbid auditd to be restarted though systemd mechanisms.)
• Multi-message events may be split into a pre-restart and post-restart portion.
• State (process table including labels) is lost across restart.

To overcome those shortcomings, the following features should be implemented:

• Signal handler
• Dump state to file (on disk or memfd). This file should contain in-flight messages, shadow process table, current configuration.
• Re-exec,
• passing the file descriptor to the new process.

[hillu]

#260 implements the state-passing part by writing a file to disk (instead of passing a file descriptor).