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.
MFC r264628: fcntl.h: Make visible various POSIX.1-2008 features.

Also, remove #if __BSD_VISIBLE where it is redundant. When __BSD_VISIBLE is
defined to 1, __POSIX_VISIBLE, __XSI_VISIBLE and __ISO_C_VISIBLE are also
defined to the newest supported version.

PR:		188173