BUG/MINOR: reload: do not fail when no socket is sent

BUG/MINOR: reload: do not fail when no socket is sent

get_old_sockets() mistakenly sets ret=0 instead of ret2=0 before leaving
when the old process announces zero FD. So it will return an error
instead of success. This must be particularly rare not to have a
single socket to offer though!

A few comments were added to make it more obvious what to expect in
return.

This must be backported to 1.8 since the bug has always been there.

(cherry picked from commit febbce87baf15b610d30172e37b98133e92159cc)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 899f15532e9770ccc3fa51b6ae237ffed8e1b5dc)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>