loader.efi(8): beef up the updating the ESP

There's a huge variety of situations when booting with UEFI. Document
more of them, hopefully better.

Feedback from: jrtc27
MFC After: 3 days
Sponsored by: Netflix

(cherry picked from commit 871911a4ab738a54a88f3533f553f5eb68c28012)
(cherry picked from commit e95ffda78ac0d61b8c17100e2061fd7542323f0d)

Approved-by: re (cperciva)