lib-renderline: expect control chars instead of <> for attributes.

lib-renderline: expect control chars instead of <> for attributes.

Using <> to distinguish attributes is clumsy as it requires that '<' be
quoted.

We already quote control characters, making them <red>^X</red> so use
control characters for distinguishing attributes.
SOH (\1) marks start of attrs
STX (\2) marks end of attrs, start of text
ETX (\3) marks end of the text for those attrs.
ACK (\6) is used as a filler when needed.

A string that start \6 or \1 is assumed not to contain meaningful <>
Otherwise we translate to remain compatible with old code.

Signed-off-by: NeilBrown <neil@brown.name>