Install the 4th files in sys/boot/forth instead of each loader

Also, move generation of loader.help into loader.mk. Set HELP_FILES=
to disable this (so we only install one help file, for now). At the
same time remove some duplicate -I lines. Fix several FILES= and
CLEANFILES= into the += form since we're touching both of those in the
.mk files. Make sure we only build one loader.help file per platform
in a unified way (we were building many on some, with the last to
install winning, though often they were the same text).

Also, we're now installing loader.rc and menu.rc everywhere. arm and
mips uboot installed these as menu.rc.sample, but there's no need
since the loader.rc for those platforms doesn't do menu.rc processing
by default. pcibios.4th is now installed everywhere, but will failsafe
on non x86 platforms (it isn't loaded by default anywhere).

These changes are too intertwined to do separately since aspects of
each are required to have a bug-free commit.

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