lua-utils: fix a few snprintf-related formatted printing warnings.
lua-utils: obey format string for object dumping.
lua-utils: properly release references from refs.priv and refs.ext.
lua-utils: removed #if 0'd code.
lua-utils: exttbl and reftbl cleanup fixes, call user destructor earlier.

Fix bad exttbl cleanup thinko (exttbl is an ordinary table, not
a table of references). Change reftbl cleanup to simply remove
our reference to reftbl. Also moved invoking the user-provided
destructor a bit earlier before reftbl and extbl are cleaned up.

Explicit cleanup of reftbl (by explicitly looping through reftb
and removing all references found there) reliably triggers an
assert in the Lua garbage-collector. I'm quite puzzled why this
happens. Maybe it has something to do with being called from the
garbage-collector... though I checked the documentation about
finalizers and did not find a word about any limitations what
one would be allowed to do from a finalizers. For the time being
we just drop our reference to reftbl, which should be good enough
as it should remove all references from the table once that is
collected. This needs further head-scratching and eyeballing...
lua-timer: fixes, explicit start/stop, removed 'data' member.

Added support for explicitly stopping and starting an otherwise
fully functional timer (IOW both interval and callback set). Also
removed the userdata-like 'data' members as pointless. Since Lua
timer is an extensible object, one can set data as an extended
Lua attribute just as well. No need to special-case it out...

Removed all attempts to self-destruct the object by explicitly
calling mrp_lua_destroy_object as Lua timer is now marked dynamic.

Timers are now automatically disabled if invoking the callback
fails.
lua-sighandler: fixes, explicit enable/disable, removed 'data' member.

Added support for explicitly enabling/disabling signal handlers.
Removed 'userdata'-like data member as pointless. Since Lua
sighander is an extensible object, one can set data as an extended
Lua attribute just as well. No need to special-case it out...

Signal handlers are now automatically disabled if invoking the
associated callback fails.
lua-deferred: fixes, explicit enable/disable, removed 'data' member.

Added support for explicitly enabling/disabling deferred callbacks.
Removed 'userdata'-like data member as pointless. Since Lua deferred
callback is an extensible object, one can set data as an extended
Lua attribute just as well. No need to special-case it out...

Deferred callbacks are now automatically disabled if invoking the
associated callback fails.
sample-config: better timer/deferred/sighandler tests.
domain-control: fixed a few compiler warnings.