keywords: don't fail on presence of "flags"

BSD file's support "flags". These have some similarity with xattr, but
for specific features, rather than general purpose key/values.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

keywords_bsd.go

@@ -0,0 +1,15 @@
+// +build darwin freebsd netbsd openbsd
+
+package mtree
+
+import (
+	"io"
+	"os"
+)
+
+var (
+	flagsKeywordFunc = func(path string, info os.FileInfo, r io.Reader) (string, error) {
+		// ideally this will pull in from here https://www.freebsd.org/cgi/man.cgi?query=chflags&sektion=2
+		return "", nil
+	}
+)