sysutils/dwatch-gource: Update to 0.8

In the network visualization code:
+ Deprecate pre-scan for existing network connections
+ Add tcp:::state-change to catch tcpdrop
+ Add missing probe ID to fbt::soreceive_dgram:entry

Reviewed by: mat (mentor)
Approved by: mat (mentor)
Sponsored by: Smule, Inc.
Differential Revision: https://reviews.freebsd.org/D15608