This allows something like the following local.lua to install a filter
to implement its own notion of hidden BEs using a naming convention of
a leading dot to hide them:

-- file: /boot/lua/local.lua
local core = require("core")

local function be_hide(be)
    if core.isSingleUserBoot() then
        -- All BEs are accepted for single-user
        return true
    end

    local name = be:match("/([^/]+)$")
    if not name then
        -- Accept malformed BEs, for whatever reason
        return true
    end

    return name:match("^%.") == nil
end

if core.bootenvFilter then
    -- Just in case we need to be compatible with older versions of
    -- core.lua without the filtering functionality.
    core.bootenvFilter(be_hide)
end
-- EOF

Requested by:	Marek Zarychta
Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D55359

PR: 293654
Reviewed by:	kevans
Approved by:	kevans
Differential Revision:	https://reviews.freebsd.org/D55713

Removing the kernel from the list of available kernels is sufficient to
avoid rendering it in the list, but we need more for booting to actually
work.  Notably, the 'kernel' loader.conf var was left intact to its
default value, so if one didn't use the kernel selector in the menu then
we'd try to boot the nonexistent 'kernel' instead of the new default
(first autodetected).

There's room to improve the error messages here, but for now let's just
make it actually work correctly.

PR:		292232
Fixes:	d04415c520b03 ("loader: lua: remove the default kernel [...]")

- s/Additionnaly/Additionally/
- s/commmand/command/

MFC after:	3 days

Reflow comments in literal text blocks
so this manual is legible at MANWIDTH 78.

MFC after:	1 day

While boot_single:lower() should always be in {yes,no}, it might not
be. In one place we check == yes, and another ~= no. Make both places ~=
no for consistency. We generally try to test != default.

Sponsored by:		Netflix

Check the number of arguments and ensure that the passed-in device to
disable parses correctly.

Sponsored by:		Netflix

Remove 'pager' shim that was last not supported in FreeBSD 12.2, which
went EOL on March 31, 2022. People have had enough time to upgrade the
boot loader.

Sponsored by:		Netflix

Make it obvious to users that the system is booting into the installer.

Reviewed by:	kevans, manu
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D51001

While these should be moved to the new format, it wasn't my intention
to force them over immediately.  Downstreams may embed their own brands
in drawer.lua, and we shouldn't break them for something like this.

Move adapt_fb_shim() up and use it for preloaded definitions to avoid
forcing the matter for now.  Perhaps in the future we'll start writing
out warnings for those that do need adapted.

Reported by:	0x1eef on IRC