Change recvattach to allow nested encryption. (see #3728)

Change recvattach to allow nested encryption. (see #3728)

* Add a FP and BODY array to the actx.  These are used to allow proper
  cleanup.

* Add HEADER and root_fp entries, to allow for index regeneration.

* Separate out the compose and recvattach index generation functions.

* Change the recvattach index generator to decrypt as encrypted parts
  are found.