sam: make current filename available to shell commands in $f (thanks aiju)

sys/src/cmd/sam/shell.c

@@ -93,6 +93,7 @@ plan9(File *f, int type, String *s, int nest)
 			close(0);	/* so it won't read from terminal */
 			open("/dev/null", 0);
 		}
+		putenv("f", Strtoc(&f->name));
 		execl(SHPATH, SH, "-c", Strtoc(&plan9cmd), nil);
 		exits("exec");
 	}