Use relations_with() for getrel(getnatp(US), THEM) where US!=THEM

Use relations_with() for getrel(getnatp(US), THEM) where US!=THEM

Replacing getrel(getnatp(US), THEM) by relations_with(US, THEM) makes
a difference only when US equals THEM.  Replace in places where it's
obvious that they're not equal.

Note: getsect() sets player->owner to "player is god or owns this
sector".  Thus, after getsect(..., &sect), sect.sct_own ==
player->cnum implies player->owner.  Conversely, !player->owner
implies sect.sct_own != player->cnum.  Similarly for getship(),
getplane() and nxtitem().