Patch: add --if-exists to pg_recvlogical

Hackers,

While doing some scripting around pg_recvlogical at $work, I found a need
for $subject. Attached, find a patch to that effect.

I tried simply to mirror the logic used elsewhere. I don't think there's
anything controversial here, but welcome any comments or suggestions.

This applies and builds successfully against master, and behaves as
designed (i.e., dropping or trying to stream from a nonexistent slot exits
cleanly). It doesn't affect any other behavior I could observe.

If accepted, this will likely need a pgindent run upon merging; I had to
give up on the rabbit hole of getting that working locally.

Thanks,

rls

--
:wq