diag_arp allow underscore in resolved host names
diag_arp allow underscore in resolved host names

is_hostname() and is_domain() allow underscore in the names. So it is possible to have underscore in host names, for example in DHCP server static mapped entries I have some things like:
10.42.3.4 client-pc-01_LAN
10.42.3.5 client-pc-01_WIFI
These reverse-resolve fine - 10.42.3.4 becomes client-pc-01_LAN
But the preg_match here misses such names that have an underscore in them.
I noticed this when looking into forum post: https://forum.pfsense.org/index.php?topic=88956.0
jxmx/pfsense,ch1c4um/pfsense,phil-davis/pfsense,phil-davis/pfsense,dennypage/pfsense,dennypage/pfsense,NOYB/pfsense,jxmx/pfsense,jxmx/pfsense,pfsense/pfsense,phil-davis/pfsense,brunostein/pfsense,dennypage/pfsense,phil-davis/pfsense,ptorsten/pfsense,ptorsten/pfsense,ptorsten/pfsense,BlackstarGroup/pfsense,brunostein/pfsense,NewEraCracker/pfsense,NewEraCracker/pfsense,pfsense/pfsense,BlackstarGroup/pfsense,NewEraCracker/pfsense,pfsense/pfsense,BlackstarGroup/pfsense,pfsense/pfsense,brunostein/pfsense,ch1c4um/pfsense,BlackstarGroup/pfsense,ch1c4um/pfsense,NOYB/pfsense,dennypage/pfsense,NewEraCracker/pfsense,ch1c4um/pfsense,NOYB/pfsense