Fix 'function declaration isn't a prototype' warning.

Pointed by:	ian
MFC after:	1 week
Simplify vntype_to_kinfo() by returning when the desired value is found
instead of breaking out of the loop and then immediately checking the loop
index so that if it was broken out of the proper value can be returned.

While here, use nitems().
Pass the length of an structure to the pretty-printer backends as a
size_t instead of an int.
Tweak pipe_truncate() to more closely match pipe_chown() and pipe_chmod()
by checking PIPE_NAMED and using invfo_truncate() for unnamed pipes.