unix/dgram: bump maximum datagram size limit to 8k

This is important for wpa_supplicant operation on a crowded network.

Note: we actually need an API to increase maximum datagram size on a
socket. Previously SO_SNDBUF magically acted like that, but that was
an undocumented "feature".

Also move the comment to the proper line. Previously it was the receive
buffer that imposed the limit. Now notion of buffer size and maximum
datagram are separate.

Reviewed by: bz, tuexen, karels
Differential Revision: https://reviews.freebsd.org/D42830
PR: 274990