Fix the behavior of exportfs -r.

Check the return of chdir(srv) to prevent exporting accidentally
the current working directory when the directory specified
with option -r doesn't exist.

Also fix at the same time the missing trailing \n in error
messages printed just before exits().

Reference: /n/sources/patch/applied/exportfs-chdir
Date: Sat Feb  9 14:22:03 CET 2013
Signed-off-by: djc@9grid.fr

--- /sys/src/cmd/exportfs/exportfs.c	Sat Feb  9 14:21:51 2013
+++ /sys/src/cmd/exportfs/exportfs.c	Sat Feb  9 14:21:46 2013
@@ -254,7 +254,12 @@
 		/* do nothing */
 	}
 	else if(srv) {
-		chdir(srv);
+		if(chdir(srv) < 0) {
+			errstr(ebuf, sizeof ebuf);
+			fprint(0, "chdir(\"%s\"): %s\n", srv, ebuf);
+			DEBUG(DFD, "chdir(\"%s\"): %s\n", srv, ebuf);
+			exits(ebuf);
+		}
 		DEBUG(DFD, "invoked as server for %s", srv);
 		strncpy(buf, srv, sizeof buf);
 	}
@@ -264,15 +269,15 @@
 		n = read(0, buf, sizeof(buf)-1);
 		if(n < 0) {
 			errstr(buf, sizeof buf);
-			fprint(0, "read(0): %s", buf);
-			DEBUG(DFD, "read(0): %s", buf);
+			fprint(0, "read(0): %s\n", buf);
+			DEBUG(DFD, "read(0): %s\n", buf);
 			exits(buf);
 		}
 		buf[n] = 0;
 		if(chdir(buf) < 0) {
 			errstr(ebuf, sizeof ebuf);
-			fprint(0, "chdir(%d:\"%s\"): %s", n, buf, ebuf);
-			DEBUG(DFD, "chdir(%d:\"%s\"): %s", n, buf, ebuf);
+			fprint(0, "chdir(%d:\"%s\"): %s\n", n, buf, ebuf);
+			DEBUG(DFD, "chdir(%d:\"%s\"): %s\n", n, buf, ebuf);
 			exits(ebuf);
 		}
 	}