make tig the process-group leader

and give it the foreground connection to the TTY.  This announces to
the OS "I am an interactive program taking input".  In the most common
case, this will already have been arranged by the user's shell.

Setting this explicitly is useful when running under the test harness
or a script like tig-pick.

This init is logically done as early as possible, as child processes
inherit these characteristics.

This also allows tig to easily ensure child process cleanup at exit.

include/tig/display.h

@@ -43,6 +43,7 @@ extern unsigned int current_view;
 #define view_is_displayed(view) \
 	(view == display[0] || view == display[1])
 
+void init_tty(void);
 void init_display(void);
 void resize_display(void);
 void redraw_display(bool clear);

src/display.c

@@ -584,7 +584,7 @@ set_terminal_modes(void)
 	leaveok(stdscr, false);
 }
 
-static void
+void
 init_tty(void)
 {
 	/* open */
@@ -598,6 +598,12 @@ init_tty(void)
 	if (!opt_tty.attr)
 		die("Failed allocation for tty attributes");
 	tcgetattr(opt_tty.fd, opt_tty.attr);
+
+	/* process-group leader */
+	signal(SIGTTOU, SIG_IGN);
+	setpgid(getpid(), getpid());
+	tcsetpgrp(opt_tty.fd, getpid());
+	signal(SIGTTOU, SIG_DFL);
 }
 
 void
@@ -607,7 +613,8 @@ init_display(void)
 	const char *term;
 	int x, y;
 
-	init_tty();
+	if (!opt_tty.file)
+		die("Can't initialize display without tty");
 
 	die_callback = done_display;
 	if (atexit(done_display))

src/tig.c

@@ -772,6 +772,8 @@ main(int argc, const char *argv[])
 	enum request request = parse_options(argc, argv, pager_mode);
 	struct view *view;
 
+	init_tty();
+
 	if (signal(SIGPIPE, SIG_IGN) == SIG_ERR)
 		die("Failed to setup signal handler");