Update tcp(4) with plpmtud blackhole sysctls

Submitted by:	Mikhail <mp@lenta.ru>
MFC after:	2 weeks
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273061 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Catch ipv6 case when attempting to do PLPMTUD blackhole detection.

Submitted by:	Mikhail <mp@lenta.ru>
MFC after:	2 weeks
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273062 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Handle small file case with regards to plpmtud blackhole detection.

Submitted by:	Mikhail <mp@lenta.ru>
MFC after:	2 weeks
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273063 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Bump .Dd

Submitted by:	gjb
MFC after:	2 weeks
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273064 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f