current_sector.php: allow forwarding to planet_main.php
current_sector.php: allow forwarding to planet_main.php

Instead of creating an error if current_sector.php is called when
the player is on a planet, simply forward to planet_main.php.

This has two primary benefits:
1. You can simply forward to current_sector.php without worrying
   if the player is on a planet.
2. The hotkey for Current Sector will also work for Planet Main.

hemberger/smr,smrealms/smr,smrealms/smr,hemberger/smr,hemberger/smr,smrealms/smr,hemberger/smr