netlink: improve nl_soreceive()

The previous commit conservatively mimiced operation of soreceive_generic().
The new code does two things:

• parses Netlink message headers and always returns at least one full nlmsg
• hides nl_buf boundaries from the userland, copying out several at once

More details can be found in the large comment block added.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D42785