Rework if_ipsec(4) to use epoch(9) instead of rmlock.

* use CK_LIST and FNV hash to keep chains of softc;
* read access to softc is protected by epoch();
* write access is protected by ipsec_ioctl_sx. Changing of softc fields
  is allowed only when softc is unlinked from CK_LIST chains.
* linking/unlinking of softc is allowed only when ipsec_ioctl_sx is
  exclusive locked.
* the plain LIST of all softc is replaced by hash table that uses ingress
  address of tunnels as a key.
* added support for appearing/disappearing of ingress address handling.
  Now it is allowed configure non-local ingress IP address, and thus the
  problem with if_ipsec(4) configuration that happens on boot, when
  ingress address is not yet configured, is solved.

Sponsored by:	Yandex LLC
Differential Revision:	https://reviews.freebsd.org/D17190

Follow the fix in r339532 (by glebius):
  Fix exiting an epoch(9) we never entered. May happen only with MAC.

Remove softc from idhash when interface is destroyed.

Add the check that current VNET is ready and access to srchash is
allowed.

ipsec_srcaddr() callback can be called during VNET teardown, since
ingress address checking subsystem isn't VNET specific. And thus
callback can make access to already freed memory. To prevent this,
use V_ipsec_idhtbl pointer as indicator of VNET readiness. And make
epoch_wait() after resetting it to NULL in vnet_ipsec_uninit() to
be sure that ipsec_srcaddr() is finished its work.

Reported by:	kp