bsdinstall: redirect stdin from /dev/null

[emaste]

Summary

In earlier pkgbase experiments pkg was not installed and the
/usr/sbin/pkg bootstrap tool stalled on [ttyin] waiting for a Y/n
response about installing pkg.

Although I do not believe this case will occur now, we do expect pkg to
be used in non-interactive mode so this will avoid future similar cases.

[ifreund_freebsdfoundation.org]

I don't think this is necessary, pkg(7) reads the ASSUME_ALWAYS_YES environment variable and should never ask for user confirmation if it is set.

If that is not happening for some reason, I believe the bug should be fixed in pkg(7) rather than worked around here.