This reverts commit 2f964c0a.

Hide a bunch of debug logging statements in #ifdef SHOW_DEBUG to prevent spinning a mutex in hot paths.

LLThread::currentID() used to return a U32, a distinct unsigned value
incremented by explicitly constructing LLThread or by calling LLThread::
registerThreadID() early in a thread launched by other means. The latter
imposed an unobvious requirement on new code based on std::thread. Using
std::thread::id instead delegates to the compiler/library the problem of
distinguishing threads launched by any means.

Change lots of explicit U32 declarations. Introduce LLThread::id_t typedef to
avoid having to run around fixing uses again if we later revisit this decision.

LLMutex, which stores an LLThread::id_t, wants a distinguished value meaning
NO_THREAD, and had an enum with that name. But as std::thread::id promises
that the default-constructed value is distinct from every valid value,
NO_THREAD becomes unnecessary and goes away.

Because LLMutex now stores LLThread::id_t instead of U32, make llmutex.h
#include "llthread.h" instead of the other way around. This makes LLMutex an
incomplete type within llthread.h, so move LLThread::lockData() and
unlockData() to the .cpp file. Similarly, remove llrefcount.h's #include
"llmutex.h" to break circularity; instead forward-declare LLMutex.

It turns out that a number of source files assumed that #include "llthread.h"
would get the definition for LLMutex. Sprinkle #include "llmutex.h" as needed.

In the SAFE_SSL code in llcorehttp/httpcommon.cpp, there's an ssl_thread_id()
callback that returns an unsigned long to the SSL library. When LLThread::
currentID() was U32, we could simply return that. But std::thread::id is very
deliberately opaque, and can't be reinterpret_cast to unsigned long.
Fortunately it can be hashed because std::hash is specialized with that type.

instead of a variable of type decltype(expression).

Using SHGetKnownFolderPath(FOLDERID_Fonts) in LLFontGL::getFontPathSystem()
requires new Windows #include files.

A variable with a constructor can't be declared within the braces of a switch
statement, even outside any of its case clauses.

Use LLStringUtil::getenv() or getoptenv() whenever we fetch a string that will
be used as a pathname.

Use LLFile::tmpdir() instead of getenv("TEMP").

As an added extra-special bonus, finally clean up $TMP/llcontrol-test-zzzzzz
directories that have been accumulating every time we run a local build!

The previous build declared a static std::ofstream; but the code that
determines the pathname for the log file is called so early that static
objects have not yet been constructed. Declare a pointer instead, and
instantiate it on demand.

This logic is essentially copy-and-edited from the same suspenders-and-belt
concerning APPDATA and CSIDL_APPDATA for SL-10153.

In that case, also update $APPDATA for child processes.