Re: Refactoring backend fork+exec code

I've now completed many of the side-quests, here are the patches that
remain.

The first three patches form a logical unit. They move the
initialization of the Port struct from postmaster to the backend
process. Currently, that work is split between the postmaster and the
backend process so that postmaster fills in the socket and some other
fields, and the backend process fills the rest after reading the startup
packet. With these patches, there is a new much smaller ClientSocket
struct that is passed from the postmaster to the child process, which
contains just the fields that postmaster initializes. The Port struct is
allocated in the child process. That makes the backend startup easier to
understand. I plan to commit those three patches next if there are no
objections.

That leaves the rest of the patches. I think they're in pretty good
shape too, and I've gotten some review on those earlier and have
addressed the comments I got so far, but would still appreciate another
round of review.

--
Heikki Linnakangas
Neon (https://neon.tech)