Replace static errno table with dynamic errname()

[Aditya-Sarna]

• Add includes for linux/string.h and linux/errname.h
• Implement lualinux_errno_index() to dynamically lookup errno values using errname()
• Keep old static errno table as fallback when CONFIG_SYMBOLIC_ERRNAME is not enabled
• Update luaopen_linux() to create errno table with metatable for dynamic lookup

When CONFIG_SYMBOLIC_ERRNAME is enabled, errno values are resolved dynamically via errname() function. Otherwise, the static table is used as before.fixes #320

[lneto]

I don't think we should use this path.. IMO, it's unnecessarily complicated. I would just add a new function to return errname, instead of carrying the burden of creating a new lib like this.. if we feel we really want a table for that, we can do it on Lua side (in the same way we do with socket/inet, for instance).