mbuf: remove remnants of caddr_t

[glebius]

Summary

caddr_t is a typedef for a character pointer (char *) that originated in
Version 7 Unix, released by AT&T in 1979. It became obsolete in 1981,
when ANSI C introduced void *.

There is a lot of code that uses m->m_data for arithmetics, thus we can't
switch it to void * easily. Let's at least stop proliferation of caddr_t.
We have developers that are twice younger than obsoletion of this type.

[glebius]

Sorry, dropped unfinished diff. Will update.

[glebius]

Change m_append, m_copyback, m_copydata prototypes to use void *.

[pouria]

We have developers that are twice younger than obsoletion of this type.

Looks like I'm not qualified to approve this one. 🚶