When opening files for write, ensure they aren't symbolic links

Issue #1048 identified that if, for example, a non privileged user
created a symbolic link from /etc/keepalvied.data to /etc/passwd,
writing to /etc/keepalived.data (which could be invoked via DBus)
would cause /etc/passwd to be overwritten.

This commit stops keepalived writing to pathnames where the ultimate
component is a symbolic link, by setting O_NOFOLLOW whenever opening
a file for writing.

This might break some setups, where, for example, /etc/keepalived.data
was a symbolic link to /home/fred/keepalived.data. If this was the case,
instead create a symbolic link from /home/fred/keepalived.data to
/tmp/keepalived.data, so that the file is still accessible via
/home/fred/keepalived.data.

There doesn't appear to be a way around this backward incompatibility,
since even checking if the pathname is a symbolic link prior to opening
for writing would create a race condition.

Signed-off-by: Quentin Armitage <quentin@armitage.org.uk>
dbus_send_reload_signal(void)
{
	gchar *path;

	if (global_connection == NULL)
		return;

	path = dbus_object_create_path_vrrp();
	dbus_emit_signal(global_connection, path, DBUS_VRRP_INTERFACE, "VrrpReloaded", NULL);
	g_free(path);
}

dbus_send_reload_signal(void)
{
	gchar *path;

	if (global_connection == NULL)
		return;

	path = dbus_object_create_path_vrrp();
	dbus_emit_signal(global_connection, path, DBUS_VRRP_INTERFACE, "VrrpReloaded", NULL);
	g_free(path);
}
