vlib: add nosyslog unix option

The "nosyslog" option disables syslog just like the "interactive" mode
but can be used together with "nodaemon".

This is useful for when VPP is running under a process supervisor like
runit or daemontools that pipe the stdout/stderr to a dedicated logger
service.

Type: feature

Change-Id: Ic4287338d6836fea9f3eabdcf960dc1f51875dd1
Signed-off-by: Ruslan Babayev <ruslan@babayev.com>
diff --git a/docs/gettingstarted/users/configuring/startup.rst b/docs/gettingstarted/users/configuring/startup.rst
index f80d55c..69d6ee5 100644
--- a/docs/gettingstarted/users/configuring/startup.rst
+++ b/docs/gettingstarted/users/configuring/startup.rst
@@ -123,6 +123,18 @@
 
    nodaemon
 
+nosyslog
+^^^^^^^^
+
+Disable syslog and log errors to stderr instead. Typical when invoking
+VPP applications from a process monitor like runit or daemontools that
+pipe service's output to a dedicated log service, which will typically
+attach a timestamp and rotate the logs as necessary.
+
+.. code-block:: console
+
+   nosyslog
+
 interactive
 ^^^^^^^^^^^