Make tbl read stdin if only flags are given.
 [rsc] --rw-rw-r-- M 1487869 glenda sys 1554 Jan 29 11:17 sys/src/cmd/tbl/t1.c
	/n/sourcesdump/2006/0129/plan9/sys/src/cmd/tbl/t1.c:43,52 - 
	/n/sourcesdump/2006/0130/plan9/sys/src/cmd/tbl/t1.c:43,49
	  	sargv = argv;
	  	sargc--; 
	  	sargv++;
	- 	if (sargc > 0) {
	- 		if(swapin() == 0)
	- 			error("flags but no input files");
	- 	} else {
	+ 	if (sargc == 0 || swapin() == 0) {
	  		tabin = (Biobuf*)getcore(sizeof(Biobuf), 1);
	  		Binit(tabin, 0, OREAD);
	  	}