MINOR: proto_sockpair: send_fd_uxst: init iobuf, cmsghdr, cmsgbuf to zeros

MINOR: proto_sockpair: send_fd_uxst: init iobuf, cmsghdr, cmsgbuf to zeros

In master-worker mode, worker process uses now send_fd_uxst() to send
'_send_status' command to master. Since refactoring, this started to trigger
the following Valgrind reports:

==810584== Syscall param sendmsg(msg.msg_iov[0]) points to uninitialised byte(s)
==810584==    at 0x4AAC99D: __libc_sendmsg (sendmsg.c:28)
==810584==    by 0x4AAC99D: sendmsg (sendmsg.c:25)
==810584==    by 0x56350F: send_fd_uxst (proto_sockpair.c:271)
==810584==    by 0x3AA25C: main (haproxy.c:4151)
==810584==  Address 0x1ffefffbfe is on thread 1's stack
==810584==  in frame #1, created by send_fd_uxst (proto_sockpair.c:241)
==810584==
==810584== Syscall param sendmsg(msg.msg_control) points to uninitialised byte(s)
==810584==    at 0x4AAC99D: __libc_sendmsg (sendmsg.c:28)
==810584==    by 0x4AAC99D: sendmsg (sendmsg.c:25)
==810584==    by 0x56350F: send_fd_uxst (proto_sockpair.c:271)
==810584==    by 0x3AA25C: main (haproxy.c:4151)
==810584==  Address 0x1ffefffc14 is on thread 1's stack
==810584==  in frame #1, created by send_fd_uxst (proto_sockpair.c:241)
==810584==

So, let's initialize with zeros all buffers, which are passed to sendmsg
syscall(), used in send_fd_uxst() to avoid these Valgrind messages. They
increase Valgrind output and could make unnoticeable some other, more important
reports.