bsdinstall: add support for graphical mode

[khorben_defora.org]

Summary

Just like for bsdconfig(8), this allows bsdinstall(8) to be started with the -X command-line parameter, thereby defaulting to Xdialog(1) in stead of bsddialog(1) to render the user interface.

With this patch, the same is also true for startbsdinstall, which in turn will start bsdinstall(8) with the -X parameter when called itself with -X.

Test Plan

# startbsdinstall -X
# bsdinstall -X