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>
vrrp_prio_handler(vector_t *strvec)
{
	vrrp_t *vrrp = LIST_TAIL_DATA(vrrp_data->vrrp);
	unsigned base_priority;

	if (!read_unsigned_strvec(strvec, 1, &base_priority, 1, VRRP_PRIO_OWNER, false)) {
		report_config_error(CONFIG_GENERAL_ERROR, "(%s) Priority not valid! must be between 1 & %d. Using default %d", vrrp->iname, VRRP_PRIO_OWNER, VRRP_PRIO_DFL);
		vrrp->base_priority = VRRP_PRIO_DFL;
	}
	else
		vrrp->base_priority = (uint8_t)base_priority;
}

vrrp_prio_handler(vector_t *strvec)
{
	vrrp_t *vrrp = LIST_TAIL_DATA(vrrp_data->vrrp);
	unsigned base_priority;

	if (!read_unsigned_strvec(strvec, 1, &base_priority, 1, VRRP_PRIO_OWNER, false)) {
		report_config_error(CONFIG_GENERAL_ERROR, "(%s) Priority not valid! must be between 1 & %d. Using default %d", vrrp->iname, VRRP_PRIO_OWNER, VRRP_PRIO_DFL);
		vrrp->base_priority = VRRP_PRIO_DFL;
	}
	else
		vrrp->base_priority = (uint8_t)base_priority;
}
