Refactor xpt_print_path, xpt_print, and xpt_path_string. Implement
all of them in terms of an sbuf-based back-end, xpt_path_sbuf. This
unifies the implementation, but more importantly it stops the output
fropm being split between 4 or more invocations of printf. The
multiple invocations cause interleaving of the messages on the
console during boot, making the output of disk discovery often
unintelligible. This change helps a lot, but more work is needed.

Reviewed by: ken, mav
Sponsored by: Netflix