liblog: Add android_set_log_frontend

android_set_log_frontend allows one to select LOGGER_DEFAULT,
LOGGER_LOGD or LOGGER_NULL, the latter allows a runtime mechanism to
turn logging on or off.  LOGGER_DEFAULT is LOGGER_LOGD.

Test: gTest liblog-unit-tests notably liblog.android_set_log_frontend
      and liblog-benchmarks before and after.
Bug: 27405083
Change-Id: I365dbee9f7d83112da0ef8d1cace748d247773c8
diff --git a/liblog/README b/liblog/README
index 610338c..922b5e3 100644
--- a/liblog/README
+++ b/liblog/README
@@ -108,6 +108,11 @@
 
        int android_log_destroy(android_log_context *ctx)
 
+       #include <log/log_frontend.h>
+
+       int android_set_log_frontend(int frontend_flag)
+       int android_get_log_frontend()
+
        Link with -llog
 
 DESCRIPTION
@@ -162,6 +167,12 @@
        when  opening  the  sub-log.    It  is  recommended  to  open  the  log
        ANDROID_LOG_RDONLY in these cases.
 
+       android_set_log_frontend() selects frontend filters. Argument is either
+       LOGGER_DEFAULT,  LOGGER_LOGD  or  LOGGER_NULL.  The  latter  drops  all
+       content   on   the   floor.     Both   android_set_log_frontend()   and
+       android_get_log_frontend()  return  the  current frontend mask,   or  a
+       negative errno for any problems.
+
 ERRORS
        If messages fail, a negative error code will be returned to the caller.
 
@@ -194,4 +205,4 @@
 
 
 
-                                  17 Oct 2016                        LIBLOG(3)
+                                  08 Feb 2017                        LIBLOG(3)