HELP_FILES is a loader only thing, so move it to loader.mk. Only
generate the help file if HELP_FILES is defined. Adjust Makefiles to
new convention. Fix a few cases where ${.CURDIR}/ was missing
resulting in missing bits from the help files.
Sponsored by: Netflix
Create an interp class. Use it to separate out the different types of
interpreters: forth and simple with function pointers rather than
via #ifdefs.
Obtained from: lua boot loader project
(via https://bsdimp@github.com/bsdimp/freebsd.git lua-bootloader)
Sponsored by: Netflix
