rework filters & mappings

config defines a single datastructure that can act both as a Map and as a Filter
(DatasetMapFilter)

Cleanup wildcard syntax along the way (also changes semantics).

zfs/zfs_test.go

@@ -17,3 +17,14 @@ func TestZFSListHandlesProducesZFSErrorOnNonZeroExit(t *testing.T) {
 	assert.True(t, ok)
 	assert.Equal(t, "error: this is a mock\n", string(zfsError.Stderr))
 }
+
+func TestDatasetPathTrimNPrefixComps(t *testing.T) {
+	p, err := NewDatasetPath("foo/bar/a/b")
+	assert.Nil(t, err)
+	p.TrimNPrefixComps(2)
+	assert.True(t, p.Equal(toDatasetPath("a/b")))
+	p.TrimNPrefixComps((2))
+	assert.True(t, p.Empty())
+	p.TrimNPrefixComps((1))
+	assert.True(t, p.Empty(), "empty trimming shouldn't do harm")
+}