+ dhcp: use ICMP for IP conflict detection

+ 'icmp_timeout_msec' YAML config setting

go.sum

@@ -56,6 +56,8 @@ github.com/shirou/gopsutil v2.18.10+incompatible h1:cy84jW6EVRPa5g9HAHrlbxMSIjBh
 github.com/shirou/gopsutil v2.18.10+incompatible/go.mod h1:5b4v6he4MtMOwMlS0TUMTu2PcXUg8+E1lC7eC3UO/RA=
 github.com/shirou/w32 v0.0.0-20160930032740-bb4de0191aa4 h1:udFKJ0aHUL60LboW/A+DfgoHVedieIzIXE8uylPue0U=
 github.com/shirou/w32 v0.0.0-20160930032740-bb4de0191aa4/go.mod h1:qsXQc7+bwAM3Q1u/4XEfrquwF8Lw7D7y5cD8CuHnfIc=
+github.com/sparrc/go-ping v0.0.0-20181106165434-ef3ab45e41b0 h1:mu7brOsdaH5Dqf93vdch+mr/0To8Sgc+yInt/jE/RJM=
+github.com/sparrc/go-ping v0.0.0-20181106165434-ef3ab45e41b0/go.mod h1:eMyUVp6f/5jnzM+3zahzl7q6UXLbgSc3MKg/+ow9QW0=
 github.com/spf13/cobra v0.0.3 h1:ZlrZ4XsMRm04Fr5pSFxBgfND2EBVa1nLpiy1stUsX/8=
 github.com/spf13/cobra v0.0.3/go.mod h1:1l0Ry5zgKvJasoi3XT1TypsSe7PqH0Sj9dhYf7v3XqQ=
 github.com/spf13/pflag v1.0.3 h1:zPAT6CGy6wXeQ7NtTnaTerfKOsV6V6F8agHXFiazDkg=