libc: add link_ntoa_r()

[ivy]

Summary

this is a re-entrant version of link_ntoa. use an in-out parameter for
the buffer size, so the user requires at most two calls to determine the
needed size.

reimplement link_ntoa using link_ntoa_r with a static buffer.

[ivy]

remove an unnecessary and frankly excessive blank line

[des]

lib/libc/net/linkaddr.3

151

lib/libc/net/linkaddr.c

35

40

42

These are out of order.

[ivy]

use .Fx in the manpage; sort linkaddr.c includes