pgsql: Improve error reporting for unsupportedeffective_io_concurrency

Improve error reporting for unsupported effective_io_concurrency setting.

Give a specific error complaining about lack of posix_fadvise() when
someone tries to set effective_io_concurrency > 0 on platforms
without that.

This probably isn't worth extensive back-patching, but I (tgl) felt
cramming it into v11 was reasonable.

James Robinson

Discussion: https://postgr.es/m/153771876450.14994.560017943128223619@wrigleys.postgresql.org
Discussion: https://postgr.es/m/A3942987-5BC7-4F05-B54D-2A0EC2914B33@jlr-photo.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2b04dfc4724970231ac338aa71e529c823fc5ff6

Modified Files
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src/backend/utils/misc/guc.c   | 10 ++++++++--
src/include/pg_config_manual.h |  4 +++-
2 files changed, 11 insertions(+), 3 deletions(-)