refactor: move some anonymous enums back to non-defs headers (#26622)

It isn't really useful to put anonymous enums only used as arguments to
functions calls in _defs.h headers, as they will only be used by a file
that calls those functions, which requires including a non-defs header.

Also move os_msg() and os_errmsg() back to message.h, as on Windows they
are actual functions instead of macros.

Also remove gettext.h and globals.h from private/helpers.h.

src/nvim/message.h

@@ -1,13 +1,36 @@
 #pragma once
 
+#include <errno.h>
 #include <stdbool.h>
 #include <stddef.h>  // IWYU pragma: keep
+#include <stdio.h>
 
 #include "nvim/ex_cmds_defs.h"  // IWYU pragma: keep
 #include "nvim/grid_defs.h"
 #include "nvim/macros_defs.h"
 #include "nvim/message_defs.h"  // IWYU pragma: export
 
+/// Types of dialogs passed to do_dialog().
+enum {
+  VIM_GENERIC   = 0,
+  VIM_ERROR     = 1,
+  VIM_WARNING   = 2,
+  VIM_INFO      = 3,
+  VIM_QUESTION  = 4,
+  VIM_LAST_TYPE = 4,  ///< sentinel value
+};
+
+/// Return values for functions like vim_dialogyesno()
+enum {
+  VIM_YES        = 2,
+  VIM_NO         = 3,
+  VIM_CANCEL     = 4,
+  VIM_ALL        = 5,
+  VIM_DISCARDALL = 6,
+};
+
+enum { MSG_HIST = 0x1000, };  ///< special attribute addition: Put message in history
+
 /// First message
 extern MessageHistoryEntry *first_msg_hist;
 /// Last message
@@ -38,3 +61,14 @@ EXTERN int msg_listdo_overwrite INIT( = 0);
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "message.h.generated.h"
 #endif
+
+// Prefer using semsg(), because perror() may send the output to the wrong
+// destination and mess up the screen.
+#define PERROR(msg) (void)semsg("%s: %s", (msg), strerror(errno))
+
+#ifndef MSWIN
+/// Headless (no UI) error message handler.
+# define os_errmsg(str)        fprintf(stderr, "%s", (str))
+/// Headless (no UI) message handler.
+# define os_msg(str)           printf("%s", (str))
+#endif