kboot: Implement symlink(2)

Linux's /dev/fd is implemented inside of /proc/self/fd, so we may need
to create a symlink to it early in boot. "/dev/fd" and "/dev/std*" might
not be strictly required for the boot loader, but should be present for
maximum flexibility.

Sponsored by: Netflix

(cherry picked from commit a4ed0eb1aa64d10feaf1e64de39eb07be43a5204)