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 using the new option.

Resolves TigerVNC#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 +519,23 @@ 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;
+
+    while ((opt = getopt(argc, argv, "D")) != -1) {
+        switch (opt) {
+        case 'D':
+            forking = 0;
+            break;
+        default:
+            usage();
+        }
     }
 
-    username = argv[1];
-    display = argv[2];
+    if ((optind > 2) || (argc != optind + 2) || (argv[optind +1][0] != ':'))
+        usage();
+
+    username = argv[argc - 2];
+    display = argv[argc - 1];
 
     if (geteuid() != 0) {
         fprintf(stderr, "This program needs to be run as root!\n");
@@ -534,8 +551,10 @@ 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 +605,8 @@ 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 +17,15 @@ 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. If the -D option is used, it does not fork and
+detach. This option is provided for use with system service managers that
+require services to run in the foreground. 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