Fix suspension capability when calling out to external commands

Clear the terminal before calling out to an external command so the
altscreen won't mess up any downstream subshell management. Restore
terminal settings as per init_display when restoring. Reproducible with
example from[0]:

 - Run `tig status`
 - Press 'e' to open Vim
 - Press Ctrl-Z to suspend Vim
 - Run `fg`
 - Quit Vim

 [0] #232 (comment)

Refs: #232

src/display.c

@@ -57,6 +57,8 @@ open_external_viewer(const char *argv[], const char *dir, bool silent, bool conf
 		ok = io_run_bg(argv, dir);
 
 	} else {
+		clear();
+		refresh();
 		endwin();                  /* restore original tty modes */
 		ok = io_run_fg(argv, dir);
 		if (confirm || !ok) {
@@ -65,6 +67,11 @@ open_external_viewer(const char *argv[], const char *dir, bool silent, bool conf
 			fprintf(stderr, "Press Enter to continue");
 			getc(opt_tty);
 		}
+		nonl();		/* Disable conversion and detect newlines from input. */
+		raw();       /* Take input chars one at a time, no wait for \n */
+		noecho();       /* Don't echo input */
+		curs_set(0);
+		leaveok(stdscr, false);
 	}
 
 	if (watch_update(WATCH_EVENT_AFTER_COMMAND) && refresh) {
@@ -463,10 +470,7 @@ init_display(void)
 		die("Failed to register done_display");
 
 	/* Initialize the curses library */
-	if (!no_display && isatty(STDIN_FILENO)) {
-		cursed = !!initscr();
-		opt_tty = stdin;
-	} else {
+	{
 		/* Leave stdin and stdout alone when acting as a pager. */
 		FILE *out_tty;