add lookup table sekeleton to build system
milestone: add lookup_table stmt to RainerScript

... but still does nothing execpt calling the handler.
Merge branch 'master' into master-lookup
milestone: lookup table ready for actual load (but load not yet done)
milestone: read in lookup table file & json parse it
milestone: completed string lookup table load (naive version)
doc: copy&paste error fix
milestone: basic lookup() functionality is present

done the naive way, string indzes only
add base plumbing for (later) dynamic table reload

among others, we change some internal interfaces. So far, we only
add the necessary locks. More work in later commits.
reload lookup tables on HUP
Merge branch 'master-lookup'

Conflicts:
	grammar/rainerscript.h
	runtime/rsyslog.h