OPTIM: quic: improve slightly qc_snd_buf() internal

OPTIM: quic: improve slightly qc_snd_buf() internal

qc_snd_buf() is a wrapper for sendmsg() syscall (or its derivatives)
used for all QUIC emissions. This patch aims at removing several
non-optimal code sections :

* fd_send_ready() for connected sockets is only checked on the function
  preambule instead of inside the emission loop

* zero-ing msghdr structure for unconnected sockets is removed. This is
  unnecessary as all fields are properly initialized then.

* extra memcpy/memset invocations when using IP_PKTINFO/IPV6_RECVPKTINFO
  are removed by setting directly the address value into cmsg buffer