[log,db] fix options to os.date() on non-posix systems

Fixes https://github.com/lualatex/luaotfload/issues/233

On non-POSIX systems, the Lua interpeter is compiled with a reduced set
of options accepted by ``os.date()`` [0].

[0] http://www.lua.org/source/5.2/loslib.c.html#LUA_STRFTIMEOPTIONS