Disable nested execute. The execute protocol is flawed and cannot be

Disable nested execute.  The execute protocol is flawed and cannot be
implemented correctly by asynchronous clients --- unless a client
waits for a prompt after sending the execute command and its argument,
it is prone to send more input before the C_EXECUTE arrives.  That
input overtakes the contents of the script file.  This is almost
certain to happen when the execute is in a script file.  Disabling
that is probably more useful and certainly less painful than
documenting this mess.  The client rejects nested execute since
servcmd.c rev. 1.42.
(EXEC): new.
(player_coms): Require it for execute.
(player_set_nstat): Set it in nstat.
(execute): Clear it in nstat.