kboot: Implement data_from_kernel to read kernel variables

Since the linux kernel doesn't expose enough of the EFI details to
userland via /sys/firmware/efi, write a routine to extract data from the
kernel directly by parsing /proc/kproc and /proc/kallsyms.

Sponsored by: Netflix
Reviewed by: kevans, jhibbits
Differential Revision: https://reviews.freebsd.org/D49868