Some sysinst issues in NetBSD-7

[David Brownlee <abs%absd.org@localhost>]

Tried a test install of NetBSD-7 and ran into three issues with the
sysinst experience

- Selecting 'Use Entire Disk' and then 'Use Existing' gives a blank
disk. Perfectly reasonable behaviour but might be nice to help users
away from this

- When installing from the usb image there does not appear to be any
way to have dhcpcd set in the generated rc.conf. I tried configuring
the network in sysinst which when down in IPv6 flames and then failed
to even configure IPv4

- Enabling xdm in sysinst seems to break login on the first virtual
terminal. Ctrl+Alt+F2 wsa able to login