API: Split YXML_DATA for content/pi/attr + remove start-of-content token

The start-of-content token (previous use of YXML_CONTENT) had to be
returned together with YXML_ELEMSTART in some cases, which resulted in
the ugly bitmask hack. The token is not strictly necessary for parsing,
and I don't think that it was even that valuable, so its been removed.

The YXML_DATA token has been split up into
YXML_(CONTENT|PICONTENT|ATTRVAL) to give the application more context as
to what kind of data it is receiving. This has the added benefit that
the application doesn't need to keep track of whether it is in a context
that it doesn't care about (e.g. in a PI) in order to handle data. If
the application is interested in element content and not in PIs, then it
can now simply ignore the YXML_PI* tokens.