nrelease: Solve the initial initrd creation better.

nrelease: Solve the initial initrd creation better.

Hard links in cd9660, while not taking up more space in the ISO or on
the CD, all have distinct inode numbers, which causes them to be treated
like individual files when copying them somewhere. This caused the initrd
to overflow when we built it from the installer after booting from the
ISO.

This commit puts initrd creation into the nrelease process instead, making
use of mkinitrd(8)'s -c option which allows us to override the contents
directory to the ${ISOROOT}'s /usr/share/initrd.

Reported-by:   opvalues
Dragonfly-bug: <https://bugs.dragonflybsd.org/issues/2798>