Redirect standard descriptors for hook/notify commands

Disconnect stdin, stdout and stderr when running an external hook or
notification command. The previous solution of appending "<&- >&- 2>&-"
to the shell command line does not work if the command includes pipes.

Use shell_exec() in notify_launch_cmd() instead of a custom (and
incomplete) reimplementation of that command.

Partially addresses GitHub issue #326.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/calcurse.h

@@ -671,7 +671,6 @@ struct nbar {
 	char datefmt[BUFSIZ];	/* format for displaying date */
 	char timefmt[BUFSIZ];	/* format for displaying time */
 	char cmd[BUFSIZ];	/* notification command */
-	const char *shell;	/* user shell to launch notif. cmd */
 	unsigned notify_all;	/* notify all appointments */
 	pthread_mutex_t mutex;
 };