Replace all instances of the typedef mbuf_t with "struct mbuf *".

The typedef mbuf_t was used for the Mac OS/X port of the code long ago.
Since this port is no longer used and the use of mbuf_t obscures what
the code does (and is not consistent with style(9)), it is no longer needed.
This patch replaces all instances of mbuf_t with "struct mbuf *", so that
it is no longer used.

This patch should not result in any semantic change.