Fix early elog(FATAL)

main() says:

/*
* Fire up essential subsystems: error and memory management
*
* Code after this point is allowed to use elog/ereport, though
* localization of messages may not work right away, and messages won't go
* anywhere but stderr until GUC settings get loaded.
*/
MemoryContextInit();

However, appending elog(ERROR, "whoops") breaks like:

$ initdb -D discard_me
FATAL: whoops
PANIC: proc_exit() called in child process
no data was returned by command ""/home/nm/sw/nopath/pghead/bin/postgres" -V"
child process was terminated by signal 6: Aborted

So does the ereport(FATAL) in ClosePostmasterPorts(). The "called in child
process" check (added in commit 97550c0 of 2023-10) reads MyProcPid, which we
set later. Three ways to fix this:

1. Call InitProcessGlobals() earlier. This could also reduce the total call
sites from 3 to 2 (main() and post-fork).

2. Move MyProcPid init out of InitProcessGlobals(), to main() and post-fork.
This has less to go wrong in back branches. While probably irrelevant,
this avoids calling pg_prng_strong_seed() in processes that will exit after
help() or GucInfoMain().

3. Revert 97550c0, as commit 3b00fdb anticipated.

I don't think the choice matters much, so here is (2).