When BGP routes are missing, DHCP packets get relayed over mgmt
interface. This results in dhcpmon alerting that DHCP packets are
not being relayed. This is PR include mgmt interface as uplink
device, and so, if DHCP packet gets relayed over mgmt interface,
regular dhcpmon alert will not be issues. Instead, dhcpmon will
check the mgmt interface counts and issue a separate alert regarding
packets travelling through mgmt network.

In addition, this PR includes the following enhancements:
1. Add SIGUSR1 handler that prints out current packet counts
2. Increase alert grace window to 3 minutes from currently 2 minutes
3. Time is now computed more accurately
4. Print vlan name before counters

signed-off-by: Tamer Ahmed <tamer.ahmed@microsoft.com>