compiler/commands: Always write usage to stdout

Usage is compiler command output, not diagnostics, so we shall output it to
stdout. Also this makes: nim -h | less or nim -h | grep ... working.

compiler/commands.nim

@@ -65,14 +65,15 @@ proc getCommandLineDesc(): string =
 
 proc helpOnError(pass: TCmdLinePass) =
   if pass == passCmd1:
-    msgWriteln(getCommandLineDesc())
+    msgWriteln(getCommandLineDesc(), {msgStdout})
     msgQuit(0)
 
 proc writeAdvancedUsage(pass: TCmdLinePass) =
   if pass == passCmd1:
     msgWriteln(`%`(HelpMessage, [VersionAsString,
                                  platform.OS[platform.hostOS].name,
-                                 CPU[platform.hostCPU].name]) & AdvancedUsage)
+                                 CPU[platform.hostCPU].name]) & AdvancedUsage,
+               {msgStdout})
     msgQuit(0)
 
 proc writeVersionInfo(pass: TCmdLinePass) =
@@ -95,7 +96,7 @@ var
 
 proc writeCommandLineUsage() =
   if not helpWritten:
-    msgWriteln(getCommandLineDesc())
+    msgWriteln(getCommandLineDesc(), {msgStdout})
     helpWritten = true
 
 proc addPrefix(switch: string): string =