Create ATTACH_CONTEXT to hold attachment index. (see #3728)

Create ATTACH_CONTEXT to hold attachment index. (see #3728)

Move the idx and idxlen into ATTACH_CONTEXT.  In subsequence patches,
this structure will hold more useful items, such as the virtual index.

The swap out is straightforward, except for:
  * update_idx() in compose.c, which post-increments the idxlen in the
    call and again in the function.
  * mutt_gen_attach_list() which doesn't need to returns the new values.