pid1: preserve current value of log level across re-{load,execution}

pid1: preserve current value of log level across re-{load,execution}

To make debugging easier, this patches allows one to change the log level and
do reload/reexec without modifying configuration permanently, which makes
debugging easier.

Indeed if one changed the log max level at runtime (via the bus or via
signals), the change was lost on the next daemon reload/reexecution.

In order to restore the original value back (set via system.conf, environment
variables or any other means), the empty string in the "LogLevel" property is
now supported as well as sending SIGRTMIN+23 signal.