BUG/MEDIUM: cfgparse: wrong argument offset after parsing server "sni" keyword

BUG/MEDIUM: cfgparse: wrong argument offset after parsing server "sni" keyword

Owen Marshall reported an issue depending on the server keywords order in the
configuration.

Working line :
  server dev1 <ip>:<port> check inter 5000 ssl verify none sni req.hdr(Host)

Non working line :
  server dev1 <ip>:<port> check inter 5000 ssl sni req.hdr(Host) verify none

Indeed, both parse_server() and srv_parse_sni() modified the current argument
offset at the same time. To fix the issue, srv_parse_sni() can work on a local
copy ot the offset, leaving parse_server() responsible of the actual value.

This fix must be backported to 1.6.