logcat: add hidden --debug flag and cleanup

Allow us to debug corrupted log messages. Fix liblog to propagate
errors correctly and repair a gtest that failed.

Test: liblog-unit-tests --gtest_filter=liblog.__android_log_bswrite_and_print___max
      logcat -b events --debug -t 50
Bug: 32903864
Change-Id: Ib4eb9f466777cd23b8b54728d36354590d07869a
diff --git a/liblog/logprint.c b/liblog/logprint.c
index 4ff7e01..da80e36 100644
--- a/liblog/logprint.c
+++ b/liblog/logprint.c
@@ -1055,6 +1055,7 @@
         outRemaining--;
         /* pretend we ate all the data to prevent log stutter */
         inCount = 0;
+        if (result > 0) result = 0;
     }
 
     /* eat the silly terminating '\n' */
@@ -1078,7 +1079,7 @@
 
     entry->message = messageBuf;
 
-    return 0;
+    return result;
 }
 
 /*