[arch-dev-public] Big changes to LVM2 in testing

[Thomas Bächler]

I discovered some new awesomeness in LVM2 (okay, not THAT new, but
still, so far unknown to me).

In our lvm2 package, I enabled lvmetad - this is a metadata caching
daemon that reacts to events from udev. I completely reorganized our
systemd and mkinitcpio lvm integration:

* lvm.service and lvm-on-crypt.service are gone.
* Once you install lvm2, lvmetad.socket and dmeventd.socket are always
active in systemd.
* If you need LVM monitoring, you need to enable lvm-monitoring.service
(recommended if you use LVM, even if you don't know if you need
monitoring). I didn't enable this by default because it would always
start lvmetad.service.
* LVM is fully hotplugged via udev. You don't need to activate anything,
LVM volumes will just work. LVM no longer requires
systemd-udev-settle.service and all the race conditions should be gone.
* LVM in mkinitcpio is also fully hotplugged, lvmwait= is now a no-op.
However, LVM in mkinitcpio now requires the udev hook.

This requires that the use_lvmetad = 1 option is set in
/etc/lvm/lvm.conf - this is now the default, but a .pacnew must be
merged if it exists.

You can restrict the hotplugging of the volume groups with the
auto_activation_volume_list option in lvm.conf. If you uncomment that,
only the listed volumes will be enabled. This is commented out by default.

WARNING: If you have any incomplete or clustered volume groups, none of
this will work yet! However, I doubt any Arch user uses that.

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