add distinct error message for fcntl failures

Author: vladak

file-locking/fcntl-locking.c

@@ -106,23 +106,23 @@ lock(int fd, struct flock *fl)
 {
 	fl->l_type = F_WRLCK;
 	if (fcntl(fd, F_SETLKW, fl) == -1)
-		err(1, "fcntl");
+		err(1, "fcntl lock W");
 }
 
 static void
 lockr(int fd, struct flock *fl)
 {
 	fl->l_type = F_RDLCK;
 	if (fcntl(fd, F_SETLKW, fl) == -1)
-		err(1, "fcntl");
+		err(1, "fcntl lock R");
 }
 
 static void
 unlock(int fd, struct flock *fl)
 {
 	fl->l_type = F_UNLCK;
 	if (fcntl(fd, F_SETLKW, fl) == -1)
-		err(1, "fcntl");
+		err(1, "fcntl unlock");
 }
 
 /*