loader: always install help files

Address two issues with current help file logic:

The existing condition prevents the common help file from being
installed when there are no additional help files defined. This results
in no loader.help on EFI platforms, for example.

Second, due to the fact that we build and install multiple loader types,
each successive install will clobber the previous loader.help. The
result is that we could lose type-specific commands, or possibly list
them in loaders that do not have such commands.

Instead, give each loader type a uniquely named help file. The EFI
loader will look for /boot/loader.help.efi, userboot will look for
/boot/loader.help.userboot, etc. The interpreter variant has no effect
on which help file is loaded.

This leaves the old /boot/loader.help unused.

Some credit for the final approach goes to Mathieu <sigsys@gmail.com>
for their version of the fix in https://reviews.freebsd.org/D22951.

Approved by: re (cperciva)
PR: 267134
Reported by: Daniel O'Connor <darius@dons.net.au>
Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28591

(cherry picked from commit 8859960436f5727f163a7b3468e08710c5e6d874)
(cherry picked from commit a2b4abce0e30e014b2c08c0bdc34b368aa9c7a6f)