net: gate the dropped-send line on the category of the send it drops

2661de1 said all the "sending: " emission sites agree on the category.
There is a fourth, and it did not: EndMessage()'s -dropmessagestest path
hardcoded "net", so under plain -debug a dropped tx announced its own drop
while the "sending: tx" it interrupts was gated off under "relay". The
knob is test-only, so nothing operational rode on it, but the claim was
false as written.

Derive it from SendLogCategory() like the other three. nPushPos may be -1
at this point, and GetMessageCommand() returns "" there, so the category
is "net" exactly as before when there is no message in progress.

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