Reduce the disc1.iso size from 850+M to just over 650M.

As a result of this change, the 'kernel-dbg.txz' distribution
is no longer provided on disc1.iso, and deselected by default
in bsdinstall(8). When 'kernel-dbg.txz' is selected, network
configuration happens before the installer proceeds, to fetch
the distribution from the mirrors.

This is a direct commit to stable/11, as there is intention
to solve this differently for 12.0-RELEASE.

Reviewed by: nwhitehorn (glanced at)
Approved by: re (hrs)
Sponsored by: The FreeBSD Foundation