Avoid multiple SetLatch() calls in procsignal_sigusr1_handler()

Hi,

Most of the multiplexed SIGUSR1 handlers are setting latch explicitly
when the procsignal_sigusr1_handler() can do that for them at the end.
These multiplexed handlers are currently being used as SIGUSR1
handlers, not as independent handlers, so no problem if SetLatch() is
removed from them. A few others do it right by saying /* latch will be
set by procsignal_sigusr1_handler */. Although, calling SetLatch() in
quick succession does no harm (it just returns if the latch was
previously set), it seems unnecessary.

I'm attaching a patch that avoids multiple SetLatch() calls.

Thoughts?

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com