threads: introduce util/threadnames, refactor thread naming

This work is prerequisite to attaching thread names to log lines and deadlock
debug utilities. This code allows setting of an "internal" threadname per
thread on platforms where thread_local is available.

This commit also moves RenameThread() out of a more general module and adds a
numeric suffix to disambiguate between threads with the same name. It
explicitly names a few main threads using the new util::ThreadRename().

src/bitcoind.cpp

@@ -19,6 +19,7 @@
 #include <util/system.h>
 #include <httpserver.h>
 #include <httprpc.h>
+#include <util/threadnames.h>
 #include <util/strencodings.h>
 #include <walletinitinterface.h>
 
@@ -64,6 +65,8 @@ static bool AppInit(int argc, char* argv[])
 
     bool fRet = false;
 
+    util::ThreadRename("init");
+
     //
     // Parameters
     //