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)