bsdinstall: obtain the dialog binary from $DIALOG

[khorben]

Summary

This reflects the same initiative for bsdconfig(8), eventually allowing to install and configure the whole system only with bsddialog(1), as opposed to a combination with dialog(1).

Sponsored by: The FreeBSD Foundation

Test Plan

# bsdinstall auto
# DISTRIBUTIONS="base kern" bsdinstall checksum
# DISTRIBUTIONS="base kern" bsdinstall fetchmissingdists
# bsdinstall finalconfig
# bsdinstall hardening
# bsdinstall jail
# bsdinstall jail /path/to/target
# bsdinstall netconfig
# bsdinstall netconfig_ipv4 em0
# bsdinstall netconfig_ipv6 em0
# bsdinstall services
# bsdinstall time
# bsdinstall services
# bsdinstall time
# BSDINSTALL_CHROOT=/ bsdinstall timezone