acme: set $samfile (same as $%) during execution

R=rsc
CC=plan9port.codebot
http://codereview.appspot.com/6854092

man/man1/acme.1

@@ -527,6 +527,8 @@ producing output in a window labeled
 .BR /home/rob/sam/+Errors .
 The environment of such commands contains the variable
 .B $%
+and
+.B $samfile
 with value set to the filename of the window in which the command is run,
 and
 .B $winid

src/cmd/acme/exec.c

@@ -1421,6 +1421,7 @@ runproc(void *argvp)
 
 		if(filename){
 			putenv("%", filename);
+			putenv("samfile", filename);
 			free(filename);
 		}
 		c->md = fsysmount(rdir, ndir, incl, nincl);