cmake: add support for "unity" builds

Aka "jumbo" or "amalgamation" builds. It means to compile all sources
per target as a single C source. This is experimental.

You can enable it by passing `-DCMAKE_UNITY_BUILD=ON` to cmake.
It requires CMake 3.16 or newer.

It makes builds (much) faster, allows for better optimizations and tends
to promote less ambiguous code.

Also add a new AppVeyor CI job and convert an existing one to use
"unity" mode (one MSVC, one MinGW), and enable it for one macOS CI job.

Fix related issues:
- add missing include guard to `easy_lock.h`.
- rename static variables and functions (and a macro) with names reused
  across sources, or shadowed by local variables.
- add an `#undef` after use.
- add a missing `#undef` before use.
- move internal definitions from `ftp.h` to `ftp.c`.
- `curl_memory.h` fixes to make it work when included repeatedly.
- stop building/linking curlx bits twice for a static-mode curl tool.
  These caused doubly defined symbols in unity builds.
- silence missing extern declarations compiler warning for ` _CRT_glob`.
- fix extern declarations for `tool_freq` and `tool_isVistaOrGreater`.
- fix colliding static symbols in debug mode: `debugtime()` and
  `statename`.
- rename `ssl_backend_data` structure to unique names for each
  TLS-backend, along with the `ssl_connect_data` struct member
  referencing them. This required adding casts for each access.
- add workaround for missing `[P]UNICODE_STRING` types in certain Windows
  builds when compiling `lib/ldap.c`. To support "unity" builds, we had
  to enable `SCHANNEL_USE_BLACKLISTS` for Schannel (a Windows
  `schannel.h` option) _globally_. This caused an indirect inclusion of
  Windows `schannel.h` from `ldap.c` via `winldap.h` to have it enabled
  as well. This requires `[P]UNICODE_STRING` types, which is apperantly
  not defined automatically (as seen with both MSVS and mingw-w64).
  This patch includes `<subauth.h>` to fix it.
  Ref: https://github.com/curl/curl/runs/13987772013
  Ref: https://dev.azure.com/daniel0244/curl/_build/results?buildId=15827&view=logs&jobId=2c9f582d-e278-56b6-4354-f38a4d851906&j=2c9f582d-e278-56b6-4354-f38a4d851906&t=90509b00-34fa-5a81-35d7-5ed9569d331c
- tweak unity builds to compile `lib/memdebug.c` separately in memory
  trace builds to avoid PP confusion.
- force-disable unity for test programs.
- do not compile and link libcurl sources to libtests _twice_ when libcurl
  is built in static mode.

KNOWN ISSUES:
- running tests with unity builds may fail in cases.
- some build configurations/env may not compile in unity mode. E.g.:
  https://ci.appveyor.com/project/curlorg/curl/builds/47230972/job/51wfesgnfuauwl8q#L250

Ref: https://github.com/libssh2/libssh2/issues/1034
Ref: https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html
Ref: https://en.wikipedia.org/wiki/Unity_build

Closes #11095

lib/multi.c

@@ -112,7 +112,7 @@ static CURLMcode multi_timeout(struct Curl_multi *multi,
 static void process_pending_handles(struct Curl_multi *multi);
 
 #ifdef DEBUGBUILD
-static const char * const statename[]={
+static const char * const multi_statename[]={
   "INIT",
   "PENDING",
   "CONNECT",
@@ -201,7 +201,7 @@ static void mstate(struct Curl_easy *data, CURLMstate state
 
     infof(data,
           "STATE: %s => %s handle %p; line %d (connection #%ld)",
-          statename[oldstate], statename[data->mstate],
+          multi_statename[oldstate], multi_statename[data->mstate],
           (void *)data, lineno, connection_id);
   }
 #endif
@@ -3798,7 +3798,7 @@ void Curl_multi_dump(struct Curl_multi *multi)
       /* only display handles that are not completed */
       fprintf(stderr, "handle %p, state %s, %d sockets\n",
               (void *)data,
-              statename[data->mstate], data->numsocks);
+              multi_statename[data->mstate], data->numsocks);
       for(i = 0; i < data->numsocks; i++) {
         curl_socket_t s = data->sockets[i];
         struct Curl_sh_entry *entry = sh_getentry(&multi->sockhash, s);