pgsql: Process session_preload_libraries within InitPostgres's transact

Process session_preload_libraries within InitPostgres's transaction.

Previously we did this after InitPostgres, at a somewhat randomly chosen
place within PostgresMain.  However, since commit a0ffa885e doing this
outside a transaction can cause a crash, if we need to check permissions
while replacing a placeholder GUC.  (Besides which, a preloaded library
could itself want to do database access within _PG_init.)

To avoid needing an additional transaction start/end in every session,
move the process_session_preload_libraries call to within InitPostgres's
transaction.  That requires teaching the code not to call it when
InitPostgres is called from somewhere other than PostgresMain, since
we don't want session_preload_libraries to affect background workers.
The most future-proof solution here seems to be to add an additional
flag parameter to InitPostgres; fortunately, we're not yet very worried
about API stability for v15.

Doing this also exposed the fact that we're currently honoring
session_preload_libraries in walsenders, even those not connected to
any database.  This seems, at minimum, a POLA violation: walsenders
are not interactive sessions.  Let's stop doing that.

(All these comments also apply to local_preload_libraries, of course.)

Per report from Gurjeet Singh (thanks also to Nathan Bossart and Kyotaro
Horiguchi for review).  Backpatch to v15 where a0ffa885e came in.

Discussion: https://postgr.es/m/CABwTF4VEpwTHhRQ+q5MiC5ucngN-whN-PdcKeufX7eLSoAfbZA@mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/31ed3cf746a61538876c66a778ec8a372a3b4ffb

Modified Files
--------------
src/backend/bootstrap/bootstrap.c   |  2 +-
src/backend/postmaster/autovacuum.c |  7 +++---
src/backend/postmaster/postmaster.c | 12 +++++++--
src/backend/tcop/postgres.c         | 12 ++++-----
src/backend/utils/init/postinit.c   | 49 +++++++++++++++++++++++++++++--------
src/include/miscadmin.h             |  7 ++++--
6 files changed, 64 insertions(+), 25 deletions(-)