Add option to run vncsession without forking and detaching

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

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

Tested on Void Linux using the new option, also tested on
Fedora without the new option.

Resolves #1649

unix/vncserver/vncsession.c

@@ -503,6 +503,14 @@ run_script(const char *username, const char *display, char **envp)
     _exit(EX_OSERR);
 }
 
+static void
+usage(void)
+{
+    fprintf(stderr, "Syntax:\n");
+    fprintf(stderr, "    vncsession [-D] <username> <display>\n");
+    _exit(EX_USAGE);
+}
+
 int
 main(int argc, char **argv)
 {
@@ -511,14 +519,26 @@ main(int argc, char **argv)
 
     const char *username, *display;
 
-    if ((argc != 3) || (argv[2][0] != ':')) {
-        fprintf(stderr, "Syntax:\n");
-        fprintf(stderr, "    %s <username> <display>\n", argv[0]);
-        return EX_USAGE;
+    int opt, forking = 1;
+    if (argc == 4) {
+        if (argv[3][0] != ':') {
+            usage();
+        }
+        while ((opt = getopt(argc, argv, "D")) != -1) {
+            switch (opt) {
+            case 'D':
+                forking = 0;
+                break;
+            default:
+                usage();
+            }
+        }
+    } else if ((argc != 3) || (argv[2][0] != ':')) {
+        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 +554,10 @@ 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 +608,8 @@ 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 +17,17 @@ 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 forking services. This option is not intended for debugging in a login
+shell from a terminal or for running
+.B vncsession
+from a terminal as an ordinary user.
+
 .SH FILES
 Several VNC-related files are found in the directory $HOME/.vnc:
 .TP