Introduce nvim namespace: Fix define guards.

Change define guards from NEOVIM_XXX_H to NVIM_XXX_H:
- Change header files.
- Change clint correct guard name calculation.

src/nvim/log.h

@@ -1,5 +1,5 @@
-#ifndef NEOVIM_LOG_H
-#define NEOVIM_LOG_H
+#ifndef NVIM_LOG_H
+#define NVIM_LOG_H
 
 #include <stdbool.h>
 
@@ -53,5 +53,5 @@ bool do_log(int log_level, const char *func_name, int line_num,
 
 #endif
 
-#endif  // NEOVIM_LOG_H
+#endif  // NVIM_LOG_H