Add sequence BIO

Add sequence BIO

It's a BIO that concatenates two other BIOs when reading.
Needed so the file parser can read the file header twice, without using
rewind(3).

(Which can't be used while piping, as it turns out.)

This allows printing a subfile from a delta or snapshot:

	$ xmlstarlet sel -t -v "//_:publish[2]" delta.xml |
		base64 --decode |
		fort --mode=print