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.

src/lib/player/player.c

@@ -261,6 +261,7 @@ execute(void)
     prexec(p);
 
     while (!failed && status()) {
+	player->nstat &= ~EXEC;
 	if (recvclient(buf, sizeof(buf)) < 0)
 	    break;
 	if (parse(buf, scanspace, player->argp, player->comtail,
@@ -278,6 +279,7 @@ execute(void)
     if (failed) {
 	while (recvclient(buf, sizeof(buf)) >= 0) ;
     }
+
     pr("Execute : %s\n", failed ? "aborted" : "terminated");
     player->eof = 0;
     return RET_OK;