Change include() so it will be able to load files with forth code,
instead of just builtins. Remove #@- from the include section of the
help file, since they don't work in the new version of include, unless
BOOT_FORTH is not defined.
Change bf_run() so it will return the result returned by ficlExec(). Also,
make bf_run() push "interpret" to be executed by ficlExec(), since ficlExec()
doesn't do it by itself. (Things worked previously because nothing
recursed through ficlExec() by the way of bf_run()).
Change/extend comments on builtin behavior.
Search for "interpret" at the end of bf_init(), so /boot/boot.4th can
provide it's own version.
Remove dead code.