loader.4th dictthreshold too small

The dictthreshold in stand/forth/loader.4th is too small
resulting in full dictionary.

Reviewed by: stevek, imp
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D44414

Remove $FreeBSD$: one-line forth tag

Remove /^\\[\s*]*\$FreeBSD\$.*$\n/

loader: Fully reset terminal settings, not just colors

Reviewed by: kevans
Reviewed by: tsoome
Approved by: mav (mentor)
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D21733

loader: provide u> and xemit words if needed

We have external interpreter (userboot.so) which may be lagging behind
with updates and may be missing u> xemit words.

Revert r332275, r332272, r332270

There's problems with them. The order of efi stuff isn't quite right,
and there's various problems. Revert until thos problems can be fixed.

Reviewed by: kevans@

loader: 332270 did use wrong path for efi.4th

The correct path is still /boot/efi.4th

loader: include efi.4th only if we do have uefi

Also simplify efi.4th.

These 4th words were an attempt to allow integration into the boot
loader scripts. However, that path won't be taken after all it
seems. Remove this code before it decays into uselessness. Also remove
build dependencies on forth no longer needed.

Move sys/boot to stand. Fix all references to new location

Sponsored by: Netflix