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/validation.h

@@ -255,7 +255,7 @@ bool LoadChainTip(const CChainParams& chainparams) EXCLUSIVE_LOCKS_REQUIRED(cs_m
 /** Unload database information */
 void UnloadBlockIndex();
 /** Run an instance of the script checking thread */
-void ThreadScriptCheck();
+void ThreadScriptCheck(int worker_num);
 /** Check whether we are doing an initial block download (synchronizing from disk or network) */
 bool IsInitialBlockDownload();
 /** Retrieve a transaction (from memory pool, or from disk, if possible) */