The switch to lualoader creates a problem with userboot: the host is inclined to build userboot with Lua, but the host userboot's interpreter must match what's available on the guest. For almost all FreeBSD guests in the wild, Lua is not yet available and a Lua-based userboot will fail.
This revision updates userboot protocol to version 5, which adds a swap_interpreter callback to request a different interpreter, and tries to determine the proper interpreter to be used based on how the guest userboot.so is compiled. This is still a bit of a guess, but it's likely the best possible guess we can make in order to get it right. The interpreter is embedded in the version string, so we can open userboot.so on the guest and hunt that down to locate the interpreter it was built with.
Using -l with bhyveload will not allow an intepreter swap, even if the loader specified happens to be a userboot with the wrong interpreter. We'll simply complain about the mismatch and bail out.
For legacy guests without the updated version string, we assume they're 4th. For new guests with the updated version string, we'll read it and swap over to the proper interpreter if it doesn't match what the userboot we're using was compiled with.
Both flavors of userboot are installed by default, userboot_4th.so and userboot_lua.so. This fixes the build WITHOUT_FORTH as a coincidence, which was broken by userboot being forced to 4th.