feat: ignore swapfile for running Nvim processes #25336

Problem:
The swapfile "E325: ATTENTION" dialog is displayed when editing a file
already open in another (running) Nvim. Usually this behavior is
annoying and irrelevant:
- "Recover" and the other options ("Open readonly", "Quit", "Abort") are
  almost never wanted.
- swapfiles are less relevant for "multi-Nvim" since 'autoread' is
  enabled by default.
  - Even less relevant if user enables 'autowrite'.

Solution:
Define a default SwapExists handler which does the following:
1. If the swapfile is owned by a running Nvim process, automatically
   chooses "(E)dit anyway" (caveat: this creates a new, extra swapfile,
   which is mostly harmless and ignored except by `:recover` or `nvim -r`.
2. Shows a 1-line "ignoring swapfile..." message.
3. Users can disable the default SwapExists handler via `autocmd! nvim_swapfile`.

src/nvim/message.c

@@ -475,7 +475,14 @@ void trunc_string(const char *s, char *buf, int room_in, int buflen)
   }
 }
 
-// Note: Caller of smsg() must check the resulting string is shorter than IOSIZE!!!
+/// Shows a printf-style message with attributes.
+///
+/// Note: Caller must check the resulting string is shorter than IOSIZE!!!
+///
+/// @see semsg
+/// @see swmsg
+///
+/// @param s printf-style format message
 int smsg(int attr, const char *s, ...)
   FUNC_ATTR_PRINTF(2, 3)
 {
@@ -757,6 +764,8 @@ void emsg_invreg(int name)
 }
 
 /// Print an error message with unknown number of arguments
+///
+/// @return whether the message was displayed
 bool semsg(const char *const fmt, ...)
   FUNC_ATTR_PRINTF(1, 2)
 {
@@ -3337,9 +3346,22 @@ void give_warning(const char *message, bool hl)
   no_wait_return--;
 }
 
-void give_warning2(const char *const message, const char *const a1, bool hl)
+/// Shows a warning, with optional highlighting.
+///
+/// @param hl enable highlighting
+/// @param fmt printf-style format message
+///
+/// @see smsg
+/// @see semsg
+void swmsg(bool hl, const char *const fmt, ...)
+  FUNC_ATTR_PRINTF(2, 3)
 {
-  vim_snprintf(IObuff, IOSIZE, message, a1);
+  va_list args;
+
+  va_start(args, fmt);
+  vim_vsnprintf(IObuff, IOSIZE, fmt, args);
+  va_end(args);
+
   give_warning(IObuff, hl);
 }