Add the ppoll() system call.
Export kern_poll() needed by an upcoming Linuxulator change.

Differential Revision:	https://reviews.freebsd.org/D1133
Reviewed by:	kib, wblock
MFC after:	1 month
Regen for r274462.
Remove redundant ip6_plen initialization.

MFC after:	1 week
Sponsored by:	Yandex LLC
Strip IP header only when we act in tunnel mode.

MFC after:	1 week
Sponsored by:	Yandex LLC