efibootmgr: Simplify make_next_boot_var_name and fix cnt == 0 case

If cnt == 0 we access element 0 unconditionally, which is out of bounds,
and then if that doesn't crash and happens to be 0 we will access
element - 1, also out of bounds, and then if that doesn't crash will add
1 to whatever junk is there and use that for the variable. On CHERI,
though, this does crash. This code is also overly complicated, with
unnecessary special cases and tracking more state than needed.

Rewrite it in a more general manner that doesn't need those special
cases and naturally works for cnt == 0.

Found by: CHERI
Reviewed by: imp
Fixes: 1285bcc833a3 ("Import Netflix's efibootmgr to help manage UEFI boot variables")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44029