Add vfs_mounted and vfs_unmounted events so that components can be informed
about mount and unmount events. This is used by Juniper to implement a more
optimal implementation of NetBSD's veriexec.

This change differs from r253224 in the following way:
o   The vfs_mounted handler is called before mountcheckdirs() and with
    newdp locked. vp is unlocked.
o   The event handlers are declared in <sys/eventhandler.h> and not in
    <sys/mount.h>. The <sys/mount.h> header is used in user land code
    that pretends to be kernel code and as such creates a very convoluted
    environment. It's hard to untangle.

Submitted by:	stevek@juniper.net
Discussed with:	pjd@
Obtained from:	Juniper Networks, Inc.