A code cleanup, pre-requisite for space-luatriggers

Make assoc.c a C file, not C++.

Remove dead code in recovery.c, read_log().

Factor out utility functions from lua/init.cc to lua/utils.c,
and use lua/utils.c in the rest of the code. The purpose
of lua/init.c is to initialize Lua subsystem in the server.

Decopule say.cc from the rest of the server, move to say.c

Decouple fiber.cc from tarantool_cfg.

Decouple opts.c from tarantool_cfg.

Introduce lbox_call() which wraps Lua errors into a Tarantool
exception and use it everywhere where we used to use lua_call().

Remove box/box_lua.cc:root_L, which was just a copy of tarantool_L.

Move plugin API implementation from lua/init.cc to src/plugin.c,
plugins must be available for JS and others too.

Remove unnecessary GNUisms from plugin.c (argh!)

Remove dead code which was trying to sandbox 'ffi' package.

Fix lbox_print() which prints to the log to look more like
normal print. We should implement say() bindings in the future,
for now make what we have work the way it should work.

Update ER_PROC_LUA error message text and remove noise from it
(imporves readability of Lua errors a bit).