man: highlight relevant lines in pam_systemd_home.so example PAM snippet

let's do this like we do in the NSS module man pages.

man/pam_systemd_home.xml

@@ -116,21 +116,21 @@
 
     <programlisting>#%PAM-1.0
 auth      sufficient pam_unix.so
--auth     sufficient pam_systemd_home.so
+<command>-auth     sufficient pam_systemd_home.so</command>
 auth      required   pam_deny.so
 
 account   required   pam_nologin.so
--account  sufficient pam_systemd_home.so
+<command>-account  sufficient pam_systemd_home.so</command>
 account   sufficient pam_unix.so
 account   required   pam_permit.so
 
--password sufficient pam_systemd_home.so
+<command>-password sufficient pam_systemd_home.so</command>
 password  sufficient pam_unix.so sha512 shadow try_first_pass try_authtok
 password  required   pam_deny.so
 
 -session  optional   pam_keyinit.so revoke
 -session  optional   pam_loginuid.so
--session  optional   pam_systemd_home.so
+<command>-session  optional   pam_systemd_home.so</command>
 -session  optional   pam_systemd.so
 session   required   pam_unix.so</programlisting>
   </refsect1>