MFC r368804: ipfw(8): Fix a few mandoc related issues

• no blank before trailing delimiter
• missing section argument: Xr inet_pton
• skipping paragraph macro: Pp before Ss
• unusual Xr order: syslogd after sysrc
• tab in filled text

There were a few multiline NAT examples which used the .Dl macro with
tabs. I converted them to .Bd, which is a more suitable macro for that case.

(cherry picked from commit bae74ca92238e7df070c22f4b53dab12f60d82ef)