pgsql: Process shared_preload_libraries in single-user mode.

Process shared_preload_libraries in single-user mode.

Without processing shared_preload_libraries, it's impossible to
recover if custom WAL resource managers are needed. It may also pose a
problem running VACUUM on a table with a custom AM, if the module
implementing the AM is expecting to be loaded by
shared_preload_libraries.

The reason this wasn't done before was just the general principle to
do fewer things in single-user mode. But it's easy enough to just set
shared_preload_libraries to empty, for the same effect.

Discussion: https://postgr.es/m/9decc18a42634f8a2f15c97a385a0f51a752f396.camel%40j-davis.com
Reviewed-by: Tom Lane, Andres Freund
Backpatch-through: 15

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6c31ac091f31171601ee00972e131d8683e7b828

Modified Files
--------------
src/backend/tcop/postgres.c | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)