Decouple beadm from zfs

• Add meson.build for libbe and beadm

Uses the system libzfs, not the one in the repo.

Build and install with:

meson builddir && cd builddir
meson compile
meson install

Run the tests after installing:

su -c "STF_SUITE=/usr/share/beadm/beadm-tests RUNFILE_DIR=/usr/share/beadm/runfiles TESTFAIL_CALLBACKS=  /usr/share/zfs/zfs-tests.sh -v -r beadm-linux.run" - vozhyk