tun/tap: merge and rename to `tuntap`

tun(4) and tap(4) share the same general management interface and have a lot
in common. Bugs exist in tap(4) that have been fixed in tun(4), and
vice-versa. Let's reduce the maintenance requirements by merging them
together and using flags to differentiate between the three interface types
(tun, tap, vmnet).

This fixes a couple of tap(4)/vmnet(4) issues right out of the gate:
- tap devices may no longer be destroyed while they're open [0]
- VIMAGE issues already addressed in tun by kp

[0] emaste had removed an easy-panic-button in r240938 due to devdrn
blocking. A naive glance over this leads me to believe that this isn't quite
complete -- destroy_devl will only block while executing d_* functions, but
doesn't block the device from being destroyed while a process has it open.
The latter is the intent of the condvar in tun, so this is "fixed" (for
certain definitions of the word -- it wasn't really broken in tap, it just
wasn't quite ideal).

ifconfig(8) also grew the ability to map an interface name to a kld, so
that `ifconfig {tun,tap}0` can continue to autoload the correct module, and
`ifconfig vmnet0 create` will now autoload the correct module. This is a
low overhead addition.

(MFC commentary)

This may get MFC'd if many bugs in tun(4)/tap(4) are discovered after this,
and how critical they are. Changes after this are likely easily MFC'd
without taking this merge, but the merge will be easier.

I have no plans to do this MFC as of now.

Reviewed by:	bcr (manpages), tuexen (testing, syzkaller/packetdrill)
Input also from:	melifaro
Relnotes:	yes
Differential Revision:	https://reviews.freebsd.org/D20044

1 files changed, 8 insertions, 0 deletions

diff --git a/UPDATING b/UPDATING
index 0f6df8dc3e81..44438de07cb7 100644
--- a/UPDATING
+++ b/UPDATING
@@ -31,6 +31,14 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 13.x IS SLOW:
 	disable the most expensive debugging functionality run
 	"ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
+20190507:
+	The tap(4) driver has been folded into tun(4), and the module has been
+	renamed to tuntap.  You should update any kld_load="if_tap" or
+	kld_load="if_tun" entries in /etc/rc.conf, if_tap_load="YES" or
+	if_tun_load="YES" entries in /boot/loader.conf to load the if_tuntap
+	module instead, and "device tap" or "device tun" entries in kernel
+	config files to select the tuntap device instead.
+
 20190418:
 	The following knobs have been added related to tradeoffs between
 	safe use of the random device and availability in the absence of