merge: upstream fixes (mutt/default)

 * Create ATTACH_CONTEXT to hold attachment index. (see #3728)
 * Change helpers functions to pass actx. (see #3728)
 * Add helpers to add and remove actx entries. (see #3728)
 * Change recvattach to allow nested encryption. (see #3728)
 * Add virtual index to actx. (see #3728)
 * Fix the expand/collapse code to use the virtual index. (see #3728)
 * Fix shared attachment functions. (see #3728)
 * Fix attachment check_traditional and extract_keys operations. (see #3728)
 * Add edit-content-type helper and warning for decrypted attachments. (closes #3728)
 * Fix parent_hdr usage in mutt_attach_reply(). (see #3728)
 * Add option to run command to query attachment mime type. (closes #2933) (closes #3959)