9892 Most consumers of be_list() do not need snapshots

In particular, the speed of beadm list is massively improved under the presence of many BE snapshots.

Note that zoneadm -z <ipkg> boot calls beadm list so this resolves https://illumos.org/issues/9885

omni# for f in `seq 1 3`; do
for> time beadm list -H > /dev/null
for> done
beadm list -H > /dev/null  5.89s user 45.26s system 99% cpu 51.159 total
beadm list -H > /dev/null  5.85s user 44.89s system 99% cpu 50.756 total
beadm list -H > /dev/null  5.91s user 45.10s system 99% cpu 51.019 total



omni# export LD_LIBRARY_PATH=/l
omni# for f in `seq 1 3`; do
time /l/beadm list -H > /dev/null
done
./beadm list -H > /dev/null  0.00s user 0.08s system 87% cpu 0.092 total
./beadm list -H > /dev/null  0.00s user 0.08s system 87% cpu 0.092 total
./beadm list -H > /dev/null  0.00s user 0.08s system 88% cpu 0.090 total