New emacsclient option to either create or reuse an existing frame.

* doc/emacs/misc.texi (emacsclient Options): Document the new option.

* doc/man/emacsclient.1: Mention the new option (bug#51374).
* etc/NEWS: Mention the new option.

* lib-src/emacsclient.c (reuse_frame): New variable.
(longopts): New option.
(decode_options): Decode the new option.
(print_help_and_exit): Document the new option.
(main): Use the new option.

lib-src/emacsclient.c

@@ -116,6 +116,9 @@ static bool eval;
 /* True means open a new frame.  --create-frame etc.  */
 static bool create_frame;
 
+/* True means reuse a frame if it already exists.  */
+static bool reuse_frame;
+
 /* The display on which Emacs should work.  --display.  */
 static char const *display;
 
@@ -165,6 +168,7 @@ static struct option const longopts[] =
   { "tty",	no_argument,       NULL, 't' },
   { "nw",	no_argument,       NULL, 't' },
   { "create-frame", no_argument,   NULL, 'c' },
+  { "reuse-frame", no_argument,   NULL, 'r' },
   { "alternate-editor", required_argument, NULL, 'a' },
   { "frame-parameters", required_argument, NULL, 'F' },
 #ifdef SOCKETS_IN_FILE_SYSTEM
@@ -551,6 +555,11 @@ decode_options (int argc, char **argv)
 	  create_frame = true;
           break;
 
+	case 'r':
+	  create_frame = true;
+	  reuse_frame = true;
+	  break;
+
 	case 'p':
 	  parent_id = optarg;
 	  create_frame = true;
@@ -647,6 +656,8 @@ The following OPTIONS are accepted:\n\
 -nw, -t, --tty 		Open a new Emacs frame on the current terminal\n\
 -c, --create-frame    	Create a new frame instead of trying to\n\
 			use the current Emacs frame\n\
+-r, --reuse-frame	Create a new frame if none exists, otherwise\n\
+			use the current Emacs frame\n\
 ", "\
 -F ALIST, --frame-parameters=ALIST\n\
 			Set the parameters of a new frame\n\
@@ -1941,7 +1952,7 @@ main (int argc, char **argv)
   if (nowait)
     send_to_emacs (emacs_socket, "-nowait ");
 
-  if (!create_frame)
+  if (!create_frame || reuse_frame)
     send_to_emacs (emacs_socket, "-current-frame ");
 
   if (display)