etcdctl: support exec on lock

The lock command is clumsy to use from the command line, needing mkfifo,
wait, etc. Instead, make like consul and support launching a command if
one is given.

etcdctl/README.md

@@ -790,7 +790,7 @@ Prints a line of JSON encoding the database hash, revision, total keys, and size
 
 ## Concurrency commands
 
-### LOCK \<lockname\>
+### LOCK \<lockname\> [command arg1 arg2 ...]
 
 LOCK acquires a distributed named mutex with a given name. Once the lock is acquired, it will be held until etcdctl is terminated.
 
@@ -798,13 +798,24 @@ LOCK acquires a distributed named mutex with a given name. Once the lock is acqu
 
 Once the lock is acquired, the result for the GET on the unique lock holder key is displayed.
 
+If a command is given, it will be launched with environment variables `ETCD_LOCK_KEY` and `ETCD_LOCK_REV` set to the lock's holder key and revision.
+
 #### Example
 
+Acquire lock with standard output display:
+
 ```bash
 ./etcdctl lock mylock
 # mylock/1234534535445
 ```
 
+Acquire lock and execute `echo lock acquired`:
+
+```bash
+./etcdctl lock mylock echo lock acquired
+# lock acquired
+```
+
 #### Remarks
 
 LOCK returns a zero exit code only if it is terminated by a signal and releases the lock.