[MAC] Add DNS reset

zsh/.mac_config.zsh_mac.example

@@ -40,3 +40,5 @@ alias python="python3"
 # Tahoe 26 WindowServer bug: windows stay interactive but can't be moved by
 # mouse or API until the window-management daemon is restarted (no logout).
 alias unstick-windows="killall WindowManager"
+
+alias resetdns="sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder"