Requirements for extra rmcause "Enter"
When I prepared the patch for #802768, I supposed that questioning too much from the unit is an unnecessary game complication, so the extra's rmreqs are checked only for unit owner and the tile (plus unit class hut_behavior outside of them that allows any number of classes with each behaviour). This, if implemented this way, should be somewhere documented since other causes' rmreqs use unit data, or maybe we should change it one or another way. Also, evaluating to RPT_CERTAIN in unit_enter_hut() fails if there are unit-targeted reqs; unit_can_*_hut() evaluate to RPT_POSSIBLE for potential calling in client. We should make the functions consistent or may just prohibit unit-targeted reqs with this rmcause (but this may limit other rmcauses, e.g. pillaging by HUT_NOTHING units).