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.