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loader: restore userboot help file

Commit e32fecd0c2c3 intended to skip installing all but one copy of
each loader variant's help file, but accidentally skipped all copies for
the userboot help file. (Other loaders install help files via the _simp
variant, but there is is no userboot_simp.)

PR: 271178
Fixes: e32fecd0c2c3 ("loader: install help files only once")
Sponsored by: The FreeBSD Foundation

loader: install help files only once

Every file should be installed exactly once by `make installworld`.
This is especially important for pkgbase.

Loader help files were being installed by each loader variant (e.g.,
the simp, lua, and 4th EFI loaders). Add a (slightly hacky) mechanism
to skip installing help files for all but one variant.

PR: 271178
Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40021

userboot: handle guest interpreter mismatches more intelligently

The switch to lualoader creates a problem with userboot: the host is
inclined to build userboot with Lua, but the host userboot's interpreter
must match what's available on the guest. For almost all FreeBSD guests in
the wild, Lua is not yet available and a Lua-based userboot will fail.

This revision updates userboot protocol to version 5, which adds a
swap_interpreter callback to request a different interpreter, and tries to
determine the proper interpreter to be used based on how the guest
/boot/loader is compiled. This is still a bit of a guess, but it's likely
the best possible guess we can make in order to get it right. The
interpreter is now embedded in the resulting executable, so we can open
/boot/loader on the guest and hunt that down to derive the interpreter it
was built with.

Using -l with bhyveload will not allow an intepreter swap, even if the
loader specified happens to be a userboot with the wrong interpreter. We'll
simply complain about the mismatch and bail out.

For legacy guests without the interpreter marker, we assume they're 4th.
For new guests with the interpreter marker, we'll read it and swap over
to the proper interpreter if it doesn't match what the userboot we're using
was compiled with.

Both flavors of userboot are installed by default, userboot_4th.so and
userboot_lua.so. This fixes the build WITHOUT_FORTH as a coincidence, which
was broken by userboot being forced to 4th.

Reviewed by: imp, jhb, araujo (earlier version)
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D16945