network: add options to tweak timeouts

network: add options to tweak timeouts

 rd.net.dhcp.retry=<cnt>
     If this option is set, dracut will try to connect via dhcp
     <cnt> times before failing. Default is 1.

 rd.net.timeout.dhcp=<arg>
     If this option is set, dhclient is called with "-timeout <arg>".

 rd.net.timeout.iflink=<seconds>
     Wait <seconds> until link shows up. Default is 60 seconds.

 rd.net.timeout.ifup=<seconds>
     Wait <seconds> until link has state "UP". Default is 20 seconds.

 rd.net.timeout.route=<seconds>
     Wait <seconds> until route shows up. Default is 20 seconds.

 rd.net.timeout.ipv6dad=<seconds>
     Wait <seconds> until IPv6 DAD is finished. Default is 50 seconds.

 rd.net.timeout.ipv6auto=<seconds>
     Wait <seconds> until IPv6 automatic addresses are assigned.
     Default is 40 seconds.

 rd.net.timeout.carrier=<seconds>
     Wait <seconds> until carrier is recognized. Default is 5 seconds.