refactor: fix includes in src/compat

Add missing includes.

Swap C headers for their C++ counterparts.

Remove pointless / unmaintainable include comments. This is even more the case
when we are actually using IWYU, as if anyone wants to see the comments they can
just get IWYU to generate them.

src/compat/stdin.cpp

@@ -2,23 +2,19 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#if defined(HAVE_CONFIG_H)
-#include <config/bitcoin-config.h>
-#endif
+#include <compat/stdin.h>
 
-#include <cstdio>       // for fileno(), stdin
+#include <cstdio>
 
 #ifdef WIN32
-#include <windows.h>    // for SetStdinEcho()
-#include <io.h>         // for isatty()
+#include <windows.h>
+#include <io.h>
 #else
-#include <termios.h>    // for SetStdinEcho()
-#include <unistd.h>     // for SetStdinEcho(), isatty()
-#include <poll.h>       // for StdinReady()
+#include <termios.h>
+#include <unistd.h>
+#include <poll.h>
 #endif
 
-#include <compat/stdin.h>
-
 // https://stackoverflow.com/questions/1413445/reading-a-password-from-stdcin
 void SetStdinEcho(bool enable)
 {