lualoader: Prepare for interception of "boot" CLI cmd

core.boot and core.autoboot may both take arguments; add a helper to cleanly
append an argstring to the given loader command.

Also provide a popFrontTable() that we'll use pop the command name off of an
argv table. We don't have the table library included, and including it is
non-trivial, so we'll implement this one function that we need in lua for
the time being.