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
Merge remote-tracking branch 'freebsd/master' into hardened/current/master
Merge branch 'hardened/current/master' into hardened/current/userlandenhanced

Conflicts:
	sys/compat/freebsd32/freebsd32_proto.h
	sys/compat/freebsd32/freebsd32_syscall.h
	sys/compat/freebsd32/freebsd32_syscalls.c
	sys/compat/freebsd32/freebsd32_sysent.c
	sys/compat/freebsd32/freebsd32_systrace_args.c
	sys/compat/freebsd32/syscalls.master
	sys/kern/init_sysent.c
	sys/kern/syscalls.c
	sys/kern/syscalls.master
	sys/kern/systrace_args.c
	sys/sys/syscall.h
	sys/sys/syscall.mk
	sys/sys/sysproto.h