MFC r337390: Bump date after r337384.

MFC r337384:

Address concerns about CPU usage while doing TCP reassembly.

Currently, the per-queue limit is a function of the receive buffer
size and the MSS. In certain cases (such as connections with large
receive buffers), the per-queue segment limit can be quite large.
Because we process segments as a linked list, large queues may not
perform acceptably.

The better long-term solution is to make the queue more efficient.
But, in the short-term, we can provide a way for a system
administrator to set the maximum queue size.

We set the default queue limit to 100. This is an effort to balance
performance with a sane resource limit. Depending on their
environment, goals, etc., an administrator may choose to modify this
limit in either direction.

Approved by: so
Security: FreeBSD-SA-18:08.tcp
Sponsored by: CVE-2018-6922

MFC r273081
brueffer: New sentence -> new line; use macros where appropriate.

MFC r276345
Mikhail <mp@lenta.ru
Correct naming of sysctl pmtud_blackhole_activated_min_mss.

Clarify some statements around PMTUD blackhole detection to make
the behavior more clear in the man page.

MFC r272720, 273061, 273062, 273063, 273064

Implement PLPMTUD blackhole detection (RFC 4821), inspired by code
from xnu sources. If we encounter a network where ICMP is blocked
the Needs Frag indicator may not propagate back to us. Attempt to
downshift the mss once to a preconfigured value.

Note, this is turned off by default.

Merge r257846:
Make TCP_KEEP* socket options readable. At least PostgreSQL wants
to read the values.

MFC r273081
brueffer: New sentence -> new line; use macros where appropriate.

MFC r276345
Mikhail <mp@lenta.ru
Correct naming of sysctl pmtud_blackhole_activated_min_mss.

Clarify some statements around PMTUD blackhole detection to make
the behavior more clear in the man page.

MFC r272720, 273061, 273062, 273063, 273064

Implement PLPMTUD blackhole detection (RFC 4821), inspired by code
from xnu sources. If we encounter a network where ICMP is blocked
the Needs Frag indicator may not propagate back to us. Attempt to
downshift the mss once to a preconfigured value.

Note, this is turned off by default.

Merge r257846:
Make TCP_KEEP* socket options readable. At least PostgreSQL wants
to read the values.