bsdinstall: handle command-line options in targets

[khorben_defora.org]

Summary

This adds a usage screen to bsdinstall(8).

This will allow adding support for graphical mode with "-X", as is already the case with bsdconfig(8).

The changes in the targets are necessary in this regard, as processing the "-X" flag in bsdconfig's dialog.subr will expect it as a parameter there.

Test Plan

$ bsdinstall -?
Illegal option -?
Usage: bsdinstall [-dD:][target][...]
  -d    Enable debugging output
  -D    Save debugging output to a file