mshow: inconsistent enumeration of MIME parts

[algor512]

The numbering of MIME parts differs in mshow and mshow -t, for example, mshow shows

From: <...>
Subject: Test
To: <...>
Date: Tue, 19 Dec 2023 17:45:46 +0100 (CET) (18 hours, 28 minutes, 29 seconds ago)

--- 1: multipart/mixed size=3224058 ---
--- --- 2: multipart/alternative size=451 ---
--- --- --- 3: text/plain size=13 ---
Test email.
--- --- 4: image/jpeg size=2175349 name="img.jpg" ---
no filter or default handler
--- --- 5: application/pdf size=210485 name="doc.pdf" ---
no filter or default handler 

while mshow -t shows

<path to file>
  1: multipart/mixed size=3224058
    2: multipart/alternative size=451
      3: text/plain size=13
      4: text/html size=166
    5: image/jpeg size=2175349 name="img.jpg"
    6: application/pdf size=210485 name="doc.pdf"

[leahneukirchen]

Duplicate of #247

This is not so easy to fix without additional work :(