Fixes for estat usage and arg parsing error messages (#26)

* Fix usage message for zil program

* Fix arg parsing error message

Author: gllghr

cmd/estat.py

@@ -93,12 +93,11 @@ def die(*args, **kwargs):
       -d LEVEL  set BCC debug level
       -e        emit the resulting eBPF script without executing it
 
-  estat zil [options]
+  estat zil [POOL]
       Provides a breakdown of time spent doing ZIL-related activities, in
       particular the time spent allocating a block and time spent waiting for
-      the write I/O to complete.
-
-      -a pool   the pool to trace
+      the write I/O to complete. If POOL is not specified, defaults to tracing
+      the pool 'domain0'.
 """
 
 
@@ -194,7 +193,10 @@ class Args:
         else:
             assert False, "unhandled option: " + opt
 
-if len(rem_args) != 1:
+if len(rem_args) == 0:
+    die("Missing duration argument")
+
+if len(rem_args) > 1:
     die("Too many arguments")
 
 try: