Allow luajit to coexist with built-in lua

[secwall]

Adds a module config "engine-name"

[dvkashapov]

Really clever, overall LGTM! Just some minor comments.
I was having an idea about handling shebang also in SCRIPT LOAD but that would require changes in core and parsing code itself in core. We also need to update README to mention that it is required now to load LuaJIT first if it is expected to use both default and LuaJIT

[zuiderkwast]

I was having an idea about handling shebang also in SCRIPT LOAD but that would require changes in core and parsing code itself in core.

Oh, so using a shebang doesn't work with EVAL and SCRIPT LOAD? I wasn't aware of that!

I think we should fix it in the core. Ricardo would know. He knows everything about scripting engines. :D

We also need to update README to mention that it is required now to load LuaJIT first if it is expected to use both default and LuaJIT

Yeah, I think the only reason to use both at the same time is to do testing and benchmarking, such as comparing them to each other.