unixL_wait return values

[daurnimator]

The current code copies the semantics from luaposix
However there are a couple of potential changes to make

• Add support for WIFCONTINUED (as in https://github.com/daurnimator/lua-spawn/blob/22685350e0f6965b0bec32dafead026aeb5ecbed/spawn/wait.c#L62)
• Copy luaL_execresult strings instead of luaposix ones (I currently have to maintain a mapping table: https://github.com/daurnimator/lua-spawn/blob/76906e5adad96635150ca63a465e5d3748614998/spawn/wait.lua#L7)

[wahern]

I don't want to break backwards compatibility but would be fine adding another wrapper, like "waitpid$execresult" or maybe "waitpid$yyyymmdd". For the latter the module could be imported like require"unix".compat"yyyymmdd" to get the improved prototypes as of a particular version.