void set_this_player(object ob)
Change the current command giver to <ob>. <ob> may be 0 if
you want to 'deactivate' the current command giver.
This efun is not privileged, therefore it should be redefined
by a nomask simul_efun which then either completely disables
the efun or at least performs some security checks.
It is easy to undermine a mudlibs security using this efun.
Introduced in 3.2.1.
LDMud 3.2.6 added the value 0 as acceptable parameter.