system/lvm2: Merge dmeventd/device-mapper subpkgs into lvm2

This makes the packages significantly saner, at the expense of pulling
in a few more MB worth of files for users who only need dmsetup or
libdevicemapper. Currently, it's not clear to users where the
development headers or man pages for these can be found, and there are a
few more minor annoyances such as the dmeventd openrc script not being
contained in an -openrc subpackage.

The /lib/libdevmapper.so symlink should be unneccessary, since there's
already a symlink in /usr/lib/.

Half the .so symlinks are in /lib/, while the other half is in
/usr/lib/, but imho fixing that isn't worth the hassle.