bsdinstall: Fix installation script splitting

This allows writing setup scripts that contain lines starting with
"#!", e.g., a shebang when creating a shell script using cat:

#!/bin/sh
echo "Populate rc.local"
cat >/etc/rc.local<<EOF
#!/bin/sh
echo booted | logger -s -t 'example'
EOF

Prevent accidentally running a setup script left behind by a
previous invocation of bsdinstall.

Reviewed by: imp, jrtc27
Differential Revision: https://reviews.freebsd.org/D43350