ndp: Prepare for pending tcpdump update

[jrm]

Summary

npd makes use of gmt2local, however that library has been removed from
tcpdump. Add utc_offset() as a substitute.

[jrm]

Should we be concerned that the long tm->tm_gmtoff will be implicitly cast as int32_t when it's returned?

[jrm]

Make utc_offset() argument match gmt2local() and make it a static function.

[emaste]

This seems OK as a minimal change.

We may want to follow tcpdump's ts_print() more closely though, in a subsequent change.

[jrm]

Assert that the value stored in tm->tm_gmtoff will fit in a int32_t.

[jhb]

Nit: s/npd/ndp/ in commit log

usr.sbin/ndp/ndp.c

187

Given this is only used with a value of 0, I would maybe go ahead and assume that and remove the argument from here entirely, that is, have it start as:

static int32_t
utc_offset(void)
{
    struct tm *tm;
    time t;

    t = time(NULL);
    tm = localtime(&t);
...

197

[jrm]

Revert back to function with no arguments. Fix typo and formatting.