check_dhcp.c merged patch from #752 (#1375)

Merged patch from #752

Sample output with exclusive/desired mode (-x):
```
# second dhcp server is detected as rogue
./check_dhcp -s 192.168.1.1 -x
CRITICAL: Received 2 DHCPOFFER(s), Rogue DHCP Server detected! Server
192.168.1.2 offered 192.168.1.100

# support for multiple dhcp servers as well
./check_dhcp -s 192.168.1.1 -s 192.168.1.50 -x
CRITICAL: Received 2 DHCPOFFER(s), Rogue DHCP Server detected! Server
192.168.1.2 offered 192.168.1.100

# both dhcp servers are correct
./check_dhcp -s 192.168.1.1 -s 192.168.1.2 -x
OK: Received 2 DHCPOFFER(s), 2 of 2 requested servers responded, max lease
time = 691200 sec.
```
You can view, comment on, or merge this pull request online at:

  https://github.com/monitoring-plugins/monitoring-plugins/pull/1375

-- Commit Summary --

  * * check_dhcp.c added patch from #752

-- File Changes --

    M plugins-root/check_dhcp.c (89)

-- Patch Links --

https://github.com/monitoring-plugins/monitoring-plugins/pull/1375.patch
https://github.com/monitoring-plugins/monitoring-plugins/pull/1375.diff

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https://github.com/monitoring-plugins/monitoring-plugins/pull/1375