[skip ci] add a CODEOWNERS file

Summary:
Convert MAINTAINERS into a Github CODEOWNERS file.  This will
automatically assign reviewers to some GH pull requests.  The conversion
is not 1:1; some committers don't have Github accounts (e.g. adrian),
some functional areas don't neatly correspond to a set of files (e.g.
kqueue), and mailing lists can't be assigned as a reviewer (e.g.
secteam@).  But it's a start.

MFC after:	2 weeks
Reviewed by:	imp
Differential Revision: https://reviews.freebsd.org/D30559

.github/CODEOWNERS

@@ -0,0 +1,101 @@
+# Please note that the content of this file is strictly advisory.
+# No locks listed here are valid.  The only strict review requirements
+# are granted by core.  These are documented in head/LOCKS and enforced
+# by svnadmin/conf/approvers.
+# 
+# The source tree is a community effort.  However, some folks go to the
+# trouble of looking after particular areas of the tree.  In return for
+# their active caretaking of the code it is polite to coordinate changes
+# with them.  This is a list of people who have expressed an interest in
+# part of the code or listed their active caretaking role so that other
+# committers can easily find somebody who is familiar with it.  The notes
+# should specify if there is a 3rd party source tree involved or other
+# things that should be kept in mind.
+# 
+# However, this is not a 'big stick', it is an offer to help and a source
+# of guidance.  It does not override the communal nature of the tree.
+# It is not a registry of 'turf' or private property.
+# 
+# ***
+# This list is prone to becoming stale quickly.  The best way to find the recent
+# maintainer of a sub-system is to check recent logs for that directory or
+# sub-system.
+# ***
+# 
+# ***
+# Maintainers are encouraged to visit:
+# https://reviews.freebsd.org/herald
+# 
+# and configure Phabricator notifications for parts of the tree which they
+# maintain.  Notifications can automatically be sent when someone proposes a
+# revision or makes a commit to the specified subtree.
+# ***
+
+autofs/	@trasz
+iscsi/	@trasz
+kqueue/	@jmgurney
+libpam/		@dag-erling
+linprocfs/	@dag-erling
+opencrypto/	@jmgurney
+openssl/	@juikim
+procfs/		@dag-erling
+pseudofs/	@dag-erling
+rctl/	@trasz
+vmm/	@bsdjhb @grehan-freebsd
+/bin/sh		@jillest
+/contrib/atf		@ngie
+/contrib/capsicum-test	@ngie
+/contrib/googletest	@ngie
+/contrib/ipfilter	@cschuber
+/contrib/libcxxrt	@DimitryAndric @emaste
+/contrib/llvm-project	@DimitryAndric
+/contrib/llvm-project/libunwind @DimitryAndric @emaste @bsdjhb
+/contrib/llvm-project/lldb @DimitryAndric @emaste
+/contrib/llvm-project/openmp @DimitryAndric @emaste
+/contrib/netbsd-tests	@ngie
+/contrib/pjdfstest	@ngie @asomers
+/crypto/openssh	@dag-erling
+/etc/mail	@gshapiro
+/etc/sendmail	@gshapiro
+/lib/libc/sys/kevent.c	@jmgurney
+/lib/libdpv	@devinteske
+/lib/libfetch	@dag-erling
+/lib/libfigpar	@dag-erling
+/lib/libvmmapi	@bsdjhb @grehan-freebsd
+/sbin/mount_fusefs @asomers
+/share/mk	@bsdimp @bapt @bdrewery @emaste @sgerraty
+/stand/forth	@devinteske
+/stand/lua	@kevans91
+/sys/amd64/pci	@bsdimp @bsdjhb
+/sys/arm/allwinner	@evadot
+/sys/arm64/rockchip	@evadot
+/sys/compat/linuxkpi	@hselasky
+/sys/contrib/dev/ice	@ricera
+/sys/contrib/ipfilter	@cschuber
+/sys/dev/drm2	@bsdimp
+/sys/dev/e1000	@ricera
+/sys/dev/ice	@ricera
+/sys/dev/ixgbe	@ricera
+/sys/dev/ixl	@ricera
+/sys/dev/ofw	@nwhitehorn
+/sys/dev/pci	@bsdimp @bsdjhb
+/sys/dev/sound/usb	@hselasky
+/sys/dev/usb	@hselasky
+/sys/fs/fuse/	@asomers
+/sys/netinet/ip_carp.c	@glebius
+/sys/netinet/sctp_*	@tuexen
+/sys/netpfil/pf	@kprovost @glebius
+/sys/x86/pci	@bsdimp @bsdjhb
+/tests	@ngie
+/tests/sys/fs/fusefs/ @asomers
+/tools/build	@bsdimp
+/tools/tools/nanobsd	@bsdimp
+/usr.bin/dpv	@devinteske
+/usr.bin/top	@grimreaper
+/usr.sbin/bhyve	@bsdjhb @grehan-freebsd
+/usr.sbin/bhyvectl	@bsdjhb @grehan-freebsd
+/usr.sbin/bhyveload	@bsdjhb @grehan-freebsd
+/usr.sbin/bsdconfig	@devinteske
+/usr.sbin/lpr	@gbergling
+/usr.sbin/sysrc	@devinteske
+/usr/bin/fetch	@dag-erling