Add option to run vncsession without forking and detaching

Option is -D, which is what sshd uses for the same option.

Also added description of the new option to the vncsession
man page.

Tested on top of up-to-date version 1.13.80,
commit 094496e (Merge pull request #1648 from TigerVNC/copyright)
on Void Linux with runit system service manager.

Also tested on top of Fedora 38 with version 1.13.1 and systemd
with Type=exec service and -D option passed to vncsession in
vncsession-start.

Resolves #1649

unix/vncserver/vncsession.c

@@ -511,14 +511,23 @@ main(int argc, char **argv)
 
     const char *username, *display;
 
-    if ((argc != 3) || (argv[2][0] != ':')) {
+    int forking = 1;
+    if (argc == 4) {
+        if ((getopt(argc, argv, "D") == 'D') && (argv[3][0] == ':'))
+            forking = 0;
+        else {
+            fprintf(stderr, "Syntax:\n");
+            fprintf(stderr, "    %s [-D] <username> <display>\n", argv[0]);
+            return EX_USAGE;
+        }
+    } else if ((argc != 3) || (argv[2][0] != ':')) {
         fprintf(stderr, "Syntax:\n");
-        fprintf(stderr, "    %s <username> <display>\n", argv[0]);
+        fprintf(stderr, "    %s [-D] <username> <display>\n", argv[0]);
         return EX_USAGE;
     }
 
-    username = argv[1];
-    display = argv[2];
+    username = argv[argc - 2];
+    display = argv[argc - 1];
 
     if (geteuid() != 0) {
         fprintf(stderr, "This program needs to be run as root!\n");
@@ -534,8 +543,10 @@ main(int argc, char **argv)
         return EX_OSERR;
     }
 
-    if (begin_daemon() == -1)
-        return EX_OSERR;
+    if (forking) {
+        if (begin_daemon() == -1)
+            return EX_OSERR;
+    }
 
     openlog("vncsession", LOG_PID, LOG_AUTH);
 
@@ -586,7 +597,8 @@ main(int argc, char **argv)
         fclose(f);
     }
 
-    finish_daemon();
+    if (forking)
+        finish_daemon();
 
     while (1) {
         int status;

unix/vncserver/vncsession.man.in

@@ -3,6 +3,7 @@
 vncsession \- start a VNC server
 .SH SYNOPSIS
 .B vncsession
+.RI [-D]
 .RI < username >
 .RI <: display# >
 .SH DESCRIPTION
@@ -16,6 +17,14 @@ appropriate options and starts a window manager on the VNC desktop.
 is rarely called directly and is normally started by the system service
 manager.
 
+.SH -D OPTION
+.B vncsession
+by default forks and detaches so it normally is a systemd Type=forking service.
+If the -D option is used, it does not fork and detach. This option is provided
+for use with other types of systemd services such as Type=simple and Type=exec
+and for use with non-systemd system service managers that are not compatible
+with Type=forking services.
+
 .SH FILES
 Several VNC-related files are found in the directory $HOME/.vnc:
 .TP