lualoader: Bring in local.lua module if it exists

Provide a way for out-of-tree users of lualoader to patch into the loader
system without having to modify our distributed scripts.

Do note that we can't really offer any API compatibility guarantees at this
time due to the evolving nature of lualoader right now.

This still has some utility as local modules may add commands at the loader
prompt without relying heavily on lualoader features- this specific
functionality is less likely to change without more careful consideration.

Reviewed by: cem (earlier version)
Differential Revision: https://reviews.freebsd.org/D14439