Remove M_MULTIDATA support
The network stack has a bunch of handling for M_MULTIDATA, which is only used by the closed source Cassini (ce) sparc driver.
We pay a tax for every mblk with this code, and it would be really really good to eliminate that tax. This has a noticeable overhead on extremely busy internet servers.
One possible way to avoid breaking ce (needs verification) would be to leave a the allocation functions in place, but have them return an error. It's likely that ce will fall back to ordinary allocb() in that case.