If <flag> is 0 this command will remove the mount from the character.
Otherwise it give the invoking character a PecoPeco (if they are a Knight
series class) or a GrandPeco (if they are a Crusader seriesclass). Unlike
'setfalcon' and 'setcart' this will not work at all if they aren't of a class
which can ride.
Note: the character needs to have the skill KN_RIDING to gain a mount

The accompanying function will return 1 if the invoking character is riding a
bird and 0 if they don't.

if (checkriding()) mes "PLEASE leave your bird outside! No riding birds on the floor here!";