Don't try to execute a Lua script.

- change /etc/default format to Lua
- change defaults format to tarantool.cfg
- don't force logger/snap_dir/wal_dir/work_dir if it's set
- minor fixes in logging
- implement fio.dirname

Conflicts:
	src/lua/fiber.cc
	src/lua/msgpackffi.lua
	test/big/iterator.result
	test/big/lua.result
	test/box/msgpack.result
	third_party/lua-yaml/lyaml.cc

Conflicts:
	third_party/luajit

Implement functions which change their effective
user id ot the one of the definer during execution.

Conflicts:
	src/box/lua/box_net.lua

Conflicts:
	src/lua/bsdsocket.cc
	src/lua/bsdsocket.lua
	test/box/bsdsocket.result
	test/box/bsdsocket.test

Fixes compatibility with third-party modules.

This patch also adds argument checking to all socket methods.