Rsyslog: support dynamic file paths (#622)

As per https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/system_administrators_guide/s1-basic_configuration_of_rsyslog#s2-Actions

lenses/rsyslog.aug

@@ -49,9 +49,11 @@ let omusrmsg = Util.del_str ":omusrmsg:" .
    File action with a specified template *)
 let file_tmpl = Syslog.file . [ label "template" . Util.del_str ";" . store Rx.word ]
 
+let dynamic = [ Util.del_str "?" . label "dynamic" . store Rx.word ]
+
 let namedpipe = Syslog.pipe . Sep.space . [ label "pipe" . store Syslog.file_r ]
 
-let action = Syslog.action | omusrmsg | file_tmpl | namedpipe
+let action = Syslog.action | omusrmsg | file_tmpl | dynamic | namedpipe
 
 (* Cannot use syslog program because rsyslog does not suppport #! *)
 let program = [ label "program" . Syslog.bang .

lenses/tests/test_rsyslog.aug

@@ -210,3 +210,15 @@ test Rsyslog.lns get "include(file=\"/etc/rsyslog.d/*.conf\" mode=\"optional\")\
   { "include"
     { "file" = "/etc/rsyslog.d/*.conf" }
     { "mode" = "optional" } }
+
+(* Dynamic file name template *)
+test Rsyslog.lns get "*.* ?DynamicFile\n" =
+  { "entry"
+    { "selector"
+      { "facility" = "*" }
+      { "level" = "*" }
+    }
+    { "action"
+      { "dynamic" = "DynamicFile" }
+    }
+  }