Backport change from trunk:
api/,backends/brass/,backends/chert/,backends/dbfactory_remote.cc,
matcher/mergepostlist.cc,matcher/remotesubmatch.cc,
net/progclient.cc: Add missing RETURN() markup for debug logging,
highlighted by warning from recent GCC.

git-svn-id: file:///home/xapian-svn/xapian/branches/1.2@18240 6b2f1b92-a4f3-0310-943f-de3cbe41748b