[LLGS] Avoid misrepresenting log lines as inferior output

[labath]

Summary

in case we are logging to stdout, any log lines from the forked child can be misconstrued to be
inferior output. To avoid this, we disable all logging immediately after forking.

I also fix the implementatoion of DisableAllLogChannels, which was a no-op before this commit.

[clayborg]

Looks good.

[ovyalov]

LGTM

lldb/trunk/source/Core/Log.cpp

Log::AutoCompleteChannelName (const char *channel_name, StringList &matches)
     }
 }
 
 void
 Log::DisableAllLogChannels (Stream *feedback_strm)
 {
     CallbackMap &callback_map = GetCallbackMap ();
     CallbackMapIter pos, end = callback_map.end();
-    const char *categories[1] = {NULL};
+    const char *categories[] = {"all", nullptr};
 
     for (pos = callback_map.begin(); pos != end; ++pos)
         pos->second.disable (categories, feedback_strm);
 
     LogChannelMap &channel_map = GetChannelMap ();
     LogChannelMapIter channel_pos, channel_end = channel_map.end();
     for (channel_pos = channel_map.begin(); channel_pos != channel_end; ++channel_pos)
         channel_pos->second->Disable (categories, feedback_strm);

lldb/trunk/source/Plugins/Process/Linux/NativeProcessLinux.cpp

enum {
         eChdirFailed,
         eExecFailed,
         eSetGidFailed
     };
 
     // Child process.
     if (pid == 0)
     {
+        // First, make sure we disable all logging. If we are logging to stdout, our logs can be
+        // mistaken for inferior output.
+        Log::DisableAllLogChannels(nullptr);
         // FIXME consider opening a pipe between parent/child and have this forked child
-        // send log info to parent re: launch status, in place of the log lines removed here.
+        // send log info to parent re: launch status.
 
         // Start tracing this child that is about to exec.
         error = PtraceWrapper(PTRACE_TRACEME, 0);
         if (error.Fail())
             exit(ePtraceFailed);
 
         // terminal has already dupped the tty descriptors to stdin/out/err.
         // This closes original fd from which they were copied (and avoids