Extend Lua unix module

[cahirwpz]

We need extra functionality:

• signal handling (signal handlers and sigaction wrapper)
• getopt (for command line parsing)
• time conversion (localtime and mktime)
• throw a pair {[err] = errno, [msg] = sys_errlist[errno]} instead of just a number
• access to environment variables through environ table

In long run we could try to integrate luaposix with our code base.

Fortunately, most libraries available at LuaRocks are MIT licensed :)

[cahirwpz]

localtime and mktime are not needed. They're implemented by os.date and os.time respectively.