src/scan.c:

src/scan.c:
	scans_active_for_addr() - walk the scan list and check if there are
	any other scans in progress for a given IP address (as specified in
	dot quad format).

	When a scan fails and negative caching is enabled, check if there
	are other scans in progress for the same address.  If not, all
	scans have failed and an entry should be added in the negcache.

	Walking the list after every scan seems inefficient but I can't see
	any other way to tell if there are no more scans active.  So, at
	the moment this is a good reason for not using negative caching.