Detect incompatible pop3 servers.
 [rsc] --rw-rw-r-- M 905070 glenda sys 13024 Dec  7 15:45 sys/src/cmd/upas/fs/pop3.c
	/n/sourcesdump/2005/1207/plan9/sys/src/cmd/upas/fs/pop3.c:168,173 - 
	/n/sourcesdump/2005/1208/plan9/sys/src/cmd/upas/fs/pop3.c:168,175
	  			hastls = 1;
	  		if(strcmp(s, "PIPELINING") == 0)
	  			pop->pipeline = 1;
	+ 		if(strcmp(s, "EXPIRE 0") == 0)
	+ 			return "server does not allow mail to be left on server";
	  	}
	  
	  	if(hastls && !pop->notls){