BUG/MINOR: hlua: hook yield does not behave as expected

BUG/MINOR: hlua: hook yield does not behave as expected

In function hlua_hook, a yieldk is performed when function is yieldable.

But the following code in that function seems to assume that the yield
never returns, which is not the case!

Moreover, Lua documentation says that in this situation the yieldk call
must immediately be followed by a return.

This patch adds a return statement after the yieldk call.
It also adds some comments and removes a needless lua_sethook call.

It could be backported to all stable versions, but it is not mandatory,
because even if it is undefined behavior this bug doesn't seem to
negatively affect lua 5.3/5.4 stacks.