holdable cursor mop-up

This patch fixes a few issues with the holdable cursor code:

- correct typo in declare.sgml

- add holdable cursors to the SQL features list. I also updated the list
to reflect Tom's improvements to FETCH. If I've done this incorrectly,
let me know.

- fix a bug involving temp file re-use: since temp files can now live
longer than their creating txn, we shouldn't reset the temp file name
counter at end-of-txn

- remove any temp files used by holdable cursors when the backend exits,
using a shmem_exit() callback

Sorry for my delay in getting this patch prepared, I've been crazy busy
with exams.

Cheers,

Neil