logd: add EXIT command

Debugging aid. Allows us to run:

$ cat -n '<STUFF>\0EXIT\0' | nc -U /dev/socket/logd

and the pipeline will exit once done. Without the EXIT command
we will have to <CTRL+C> out to terminate the pipeline. This is
necessary since Android's netcat command does not support the -q
feature, and having the EXIT command is faster and cleaner anyways.

Test: manual as noted above.
Bug: 31456426
Change-Id: I65333358188db85e2eb1bb6a14deed1565826ac4