pgsql: Move code for backend startup to separate file

Move code for backend startup to separate file

This is code that runs in the backend process after forking, rather
than postmaster. Move it out of postmaster.c for clarity.

Reviewed-by: Tristan Partin, Andres Freund
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/05c3980e7f473ac2061dad9bbb7a9f0ede0279d9

Modified Files
--------------
src/backend/postmaster/launch_backend.c | 1 +
src/backend/postmaster/postmaster.c | 762 +------------------------------
src/backend/tcop/Makefile | 1 +
src/backend/tcop/backend_startup.c | 778 ++++++++++++++++++++++++++++++++
src/backend/tcop/meson.build | 1 +
src/include/postmaster/postmaster.h | 4 +-
src/include/tcop/backend_startup.h | 41 ++
7 files changed, 829 insertions(+), 759 deletions(-)