init+index: support --skip-docdata for Xapian

init+index: support --skip-docdata for Xapian

Since we no longer read document data from Xapian, allow users
to opt-out of storing it.

This breaks compatibility with previous releases of
public-inbox, but gives us a ~1.5% space savings on Xapian
storage (and associated I/O and page cache pressure reduction).