Fix copyright, remove "all rights reserved".

The eventfd code was written by me, rdivacky@ copyrigth applicable only
to epoll part of the Linuxulator code. Roman is ok to retire his copyright
from sys/kern/sys_eventfd.c and 'All rights reserved.' lines from
sys/compat/linux/linux_event.[c|h] and sys/kern/sys_eventfd.c files.

Reviewed by: kib, emaste
Approved by: rdivacky
Differential Revision: https://reviews.freebsd.org/D30677
MFC after: 2 weeks

(cherry picked from commit f570a6723e145ae585bfb402f3f42260687666d3)