Finally change the mbuf to have its own fib field instead of stealing
4 flag bits. This was supposed to happen in 8.0, and again in 2012..

MFC after:	never
Add a handy function for truncating variables to a specific byte-length. It
should be noted that newlines are both preserved and included in said byte-
count. If you want to truncate single-line values without regard to line
termination, there's always f_substr() which already exists herein.
Fix a typo in a comment.