lua: implement "luaclient".

This allows for starting Lua scripts in neutrino context from the
command line. Experimental, not fully regression-tested.
luaserver: reorganize
neutrino.cpp: omit deleting luaserver at shutdown
system/luaserver: fix destructor
system/luaserver: Terminate Lua script when luaclient dies.
system/luaserver: include poll.h explicitly
- luaserver.cpp: change testing example
Merge remote-tracking branch 'cool-cst-next/cst-next'