sockets: enable protocol specific socket buffers

Split struct sockbuf into common shared fields and protocol specific
union, where protocols are free to implement whatever buffer they
want. Such protocols should mark themselves with PR_SOCKBUF and are
expected to initialize their buffers in their pr_attach and tear
them down in pr_detach.

Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D35299