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.