pgsql: Redefine max_files_per_process to control additionally opened fi

Redefine max_files_per_process to control additionally opened files

Until now max_files_per_process=N limited each backend to open N files in
total (minus a safety factor), even if there were already more files opened in
postmaster and inherited by backends. Change max_files_per_process to control
how many additional files each process is allowed to open.

The main motivation for this is the patch to add io_method=io_uring, which
needs to open one file for each backend. Without this patch, even if
RLIMIT_NOFILE is high enough, postmaster will fail in set_max_safe_fds() if
started with a high max_connections. The cause of the failure is that, until
now, set_max_safe_fds() subtracted the already open files from
max_files_per_process.

Reviewed-by: Noah Misch <noah(at)leadboat(dot)com>
Discussion: https://postgr.es/m/w6uiicyou7hzq47mbyejubtcyb2rngkkf45fk4q7inue5kfbeo@bbfad3qyubvs
Discussion: https://postgr.es/m/CAGECzQQh6VSy3KG4pN1d=h9J=D1rStFCMR+t7yh_Kwj-g87aLQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/adb5f85fa5a00d6d5759df55feb16dc22b0fc8d7

Modified Files
--------------
doc/src/sgml/config.sgml | 8 ++++++--
src/backend/storage/file/fd.c | 14 ++++++++------
src/backend/utils/misc/guc_tables.c | 2 +-
3 files changed, 15 insertions(+), 9 deletions(-)