tdb2: add a capability list from the header.

This allows even more extensibility in future: in particular, the top
bits of each capability tell us what to do if we don't understand it:
fail the open, fail to open for write, or don't try to check the
format.

tdb_check needs to understand the capability list so it can know to
skip over it: each element in the list is prefixed with the type tag
and the length.