BUG/MEDIUM: unix: never unlink a unix socket from the file system

BUG/MEDIUM: unix: never unlink a unix socket from the file system

James Brown reported some cases where a race condition happens between
the old and the new processes resulting in the leaving process removing
a newly bound unix socket. Jeff gave all the details he observed here :

   https://www.mail-archive.com/haproxy@formilux.org/msg25001.html

The unix socket removal was an attempt at an optimal cleanup, which
almost never works anyway since the process is supposed to be chrooted.
And in the rare cases where it works it occasionally creates trouble.
There was already a workaround in place to avoid removing this socket
when it's been inherited from a parent's file descriptor.

So let's finally kill this useless stuff now to definitely get rid of
this persistent problem.

This fix should be backported to all stable releases.