services/agent/v23agentd: break into subcommands

'stop' becomes its own command

The credentials are now specified as a flag (instead of positional
argument), and default to V23_CREDENTIALS.

MultiPart: 2/2
Change-Id: I4f71ee69e05f293118c095d7acc94d53ec794a05
diff --git a/content/tutorials/security/agent.md b/content/tutorials/security/agent.md
index 89715b5..66d3c75 100644
--- a/content/tutorials/security/agent.md
+++ b/content/tutorials/security/agent.md
@@ -61,7 +61,7 @@
 to the client.
 
 Launching an agent to serve credentials in a given directory (by
-invoking `v23agentd {directory}`) sets up a socket file in the
+invoking `v23agentd --credentials={directory}`) sets up a socket file in the
 directory, which the client connects to when using `--v23.credentials
 {directory}`.  This happens in the Vanadium runtime - no new client
 code required.
@@ -95,7 +95,7 @@
 /bin/rm -f $V_TUT/server.txt
 
 # Run an agent for server credentials.
-$V_BIN/v23agentd $V_TUT/cred/alice
+$V_BIN/v23agentd --credentials=$V_TUT/cred/alice
 $V_TUT/bin/server \
     --v23.credentials $V_TUT/cred/alice \
     --endpoint-file-name $V_TUT/server.txt \
@@ -108,15 +108,15 @@
 sleep 2s
 
 # Run an agent for client credentials.
-$V_BIN/v23agentd $V_TUT/cred/bob
+$V_BIN/v23agentd --credentials=$V_TUT/cred/bob
 $V_TUT/bin/client \
     --v23.credentials $V_TUT/cred/bob \
     --server `cat $V_TUT/server.txt`
 
 # All done, kill the server and agents
 kill_tut_process TUT_PID_SERVER
-$V_BIN/v23agentd --stop $V_TUT/cred/alice
-$V_BIN/v23agentd --stop $V_TUT/cred/bob
+$V_BIN/v23agentd stop --credentials=$V_TUT/cred/alice
+$V_BIN/v23agentd stop --credentials=$V_TUT/cred/bob
 ```
 
 The above should run without errors, i.e. the client should report a