gptboot.efi: Add man page

Add a man page for gptboot.efi. Describe when and how to use this as it differs
from the BIOS cases. Include cross reference for the preferred method described
in efibootmgr(8) as well as cross links in both gptboot(8) and gptboot.efi(8) to
the other.

This man page was heavily copied from the gptboot.8 man page by Warren Block.
They are different enough to need separate man pages for clarity, but there's
enough similarity that I worry about the duplication. In the really long term,
gptboot(8) will disappear, so having the same info here will help when that
day comes. In the short to medium term, the information is likely to not
change in gptboot(8) and any changes to gptboot.efi(8) will be easier to
make in a separate copy.

loader.efi(8) needs a complete rewrite from scratch, otherwise I'd have
referenced gptboot.efi(8) from there.

Suggetions from: cress@, mhorne@
Reviewed by: rpokala@
Differential Revision: https://reviews.freebsd.org/D29591