Don't pop last context in context stack.
Avoid superfluous table lookup.

We find the default attribute in ParseSyntaxFiles and embed it
directly in the parser, rather than looking it up at runtime.
This simplifies the parsers and should boost performance.
Don't pop context on line end if we're already at root context.

This helps with the erlang.xml file, which has some odd
`#pop`s in it.