player: make "map" a system-only command

"map" was registerered as a global player command, everywhere.
However, a sanity assertion check enforced that a player could only
use it in a system (and not in planetary orbit or while docked), or
yastg would abort with:

yastg: player.c:62: cmd_map: Assertion `ship->postype == SYSTEM' failed.
Merge pull request #5 from gonzzor/map_command

player: make "map" a system-only command