POD: Fix item numbering

Newer pod2man considers =item [1-9] part of a numbered list, while =item
0 starts an unnumbered list. Add a zero effect formatting mark to override
this.

doc/apps/smime.pod around line 315: Expected text after =item, not a
number
...

PR#3146
Fixed missing =back commands
typo in SSL_get_peer_cert_chain docs

RT: 3304