mappings: fix aliasing bug with '<' wildcards

In contrast to any 'something<' mapping, a '<' mapping cannot be unique
Thus, '<' mappings are thus just an append to target, which is exactly
what we get when trimming empty prefix ''.

Otherwise, given mapping

{ "<": "storage/backups/app-srv" }

Before (clearly a conflict)
zroot     => storage/backups/app-srv
storage   => storage/backups/app-srv
After:
zroot     => storage/backups/app-srv/zroot
storage   => storage/backups/app-srv/storage

However, mapping directly with subtree wildcard is still possible, just
not with the root wildcard

{
    "<"              "storage/backups/app-srv"
    "zroot/var/db<": "storage/db_replication/app-srv"
}

fixes #22

cmd/config_mapfilter.go

@@ -125,14 +125,9 @@ func (m DatasetMapFilter) Map(source *zfs.DatasetPath) (target *zfs.DatasetPath,
 		return
 	}
 	if me.subtreeMatch {
-		// strip common prefix
+		// strip common prefix ('<' wildcards are no special case here)
 		extendComps := source.Copy()
-		if me.path.Empty() {
-			// special case: trying to map the root => strip first component
-			extendComps.TrimNPrefixComps(1)
-		} else {
-			extendComps.TrimPrefix(me.path)
-		}
+		extendComps.TrimPrefix(me.path)
 		target.Extend(extendComps)
 	}
 	return

cmd/config_test.go

@@ -129,8 +129,8 @@ func TestDatasetMapFilter(t *testing.T) {
 	expectMapping(map1, "a/b/c/d/e", "")
 	expectMapping(map1, "a/b/e", "root2/e")
 	expectMapping(map1, "a/b", "root2")
-	expectMapping(map1, "x", "root3/b/c")
-	expectMapping(map1, "x/y", "root3/b/c/y")
+	expectMapping(map1, "x", "root3/b/c/x")
+	expectMapping(map1, "x/y", "root3/b/c/x/y")
 	expectMapping(map1, "q", "root4/1/2")
 	expectMapping(map1, "b", "")
 	expectMapping(map1, "q/r", "root4/1/2/r")