os: Add OS_STD*_FILENO constants

src/nvim/os/os_defs.h

@@ -13,6 +13,13 @@
 # include "nvim/os/unix_defs.h"
 #endif
 
+/// File descriptor number used for standard IO streams
+enum {
+  OS_STDIN_FILENO = STDIN_FILENO,
+  OS_STDOUT_FILENO = STDOUT_FILENO,
+  OS_STDERR_FILENO = STDERR_FILENO,
+};
+
 #define BASENAMELEN (NAME_MAX - 5)
 
 // Use the system path length if it makes sense.

src/nvim/os/win_defs.h

@@ -91,4 +91,14 @@ typedef SSIZE_T ssize_t;
 # define S_ISCHR(m) (((m) & S_IFMT) == S_IFCHR)
 #endif
 
+#ifndef STDIN_FILENO
+# define STDIN_FILENO 0
+#endif
+#ifndef STDOUT_FILENO
+# define STDOUT_FILENO 1
+#endif
+#ifndef STDERR_FILENO
+# define STDERR_FILENO 2
+#endif
+
 #endif  // NVIM_OS_WIN_DEFS_H