eventlog: Repost initialisation events when eventlog restarts

If startEventlog is called after the program has already started running
then quite a few useful events are missing from the eventlog because
they are only posted when the program starts. This patch adds a
mechanism to declare that an event should be reposted everytime the
startEventlog function is called.

Now in EventLog.c there is a global list of functions called
`eventlog_header_funcs` which stores a list of functions which should be
called everytime the eventlog starts.

When calling `postInitEvent`, the event will not only be immediately
posted to the eventlog but also added to the global list.

When startEventLog is called, the list is traversed and the events
reposted.