Add --esp/-E argument to print the currently booted ESP

Add code to decode the BootCurrent and BootXXXX variable it points at
to deduce the ESP used to boot the system. By default, it prints the
path to that device. With --unix-path (-p) it will instead print the
current mount point for the ESP, if any (or an error). With
--device-path (-d) it wil print the UEFI device path for the ESP.

Note: This is the best guess based on the UEFI variables. If the ESP
is part of a gmirror, etc, that won't be reported. If by some weird
chance there was a complicated series of chain boots, this may not be
what you want. For setups that don't add layers on top of the raw
devices, it is accurate.

Differential Revision: https://reviews.freebsd.org/D22432