logd: add getEventTag command and service

Will register a new event tag by name and format, and return an
event-log-tags format response with the newly allocated tag.
If format is not specified, then nothing will be recorded, but
a pre-existing named entry will be listed. If name and format are
not specified, list all dynamic entries. If name=* list all
event log tag entries.

Stickiness through logd crash will be managed with the tmpfs file
/dev/event-log-tags and through a reboot with add_tag entries in
the pmsg last logcat event log. On debug builds we retain a
/data/misc/logd/event-log-tags file that aids stickiness and that
can be picked up by the bugreport.

If we detect truncation damage to /dev/event-log-tags, or to
/data/misc/logd/event-log-tags, rebuild file with a new first line
signature incorporating the time so mmap'd readers of the file can
detect the possible change in shape and order.

Manual testing:

Make sure nc (netcat) is built for the target platform on the host:
$ m nc

Then the following can be used to issue a request on the platform:
$ echo -n 'getEventTag name=<name> format="<format>"\0EXIT\0' |
> nc -U /dev/socket/logd

Test: gTest logd-unit-test --gtest_filter=getEventTag*
Bug: 31456426
Change-Id: I5dacc5f84a24d52dae09cca5ee1a3a9f9207f06d
diff --git a/logd/main.cpp b/logd/main.cpp
index 5878f15..2551f2e 100644
--- a/logd/main.cpp
+++ b/logd/main.cpp
@@ -244,7 +244,7 @@
     // anything else, we have even lesser privileges and accept our fate. Not
     // worth checking for error returns setting this thread's privileges.
     (void)setgid(AID_SYSTEM); // readonly access to /data/system/packages.list
-    (void)setuid(AID_LOGD);   // access to everything logd.
+    (void)setuid(AID_LOGD);   // access to everything logd, eg /data/misc/logd
 
     while (reinit_running && !sem_wait(&reinit) && reinit_running) {
 
@@ -271,6 +271,7 @@
             logBuf->init();
             logBuf->initPrune(NULL);
         }
+        android::ReReadEventLogTags();
     }
 
     return NULL;
@@ -304,24 +305,6 @@
     sem_post(&reinit);
 }
 
-// tagToName converts an events tag into a name
-const char *android::tagToName(size_t *len, uint32_t tag) {
-    static const EventTagMap *map;
-
-    if (!map) {
-        sem_wait(&sem_name);
-        if (!map) {
-            map = android_openEventTagMap(NULL);
-        }
-        sem_post(&sem_name);
-        if (!map) {
-            if (len) len = 0;
-            return NULL;
-        }
-    }
-    return android_lookupEventTag_len(map, len, tag);
-}
-
 static void readDmesg(LogAudit *al, LogKlog *kl) {
     if (!al && !kl) {
         return;