pgsql: IWYU widely useful pragmas

IWYU widely useful pragmas

Add various widely useful "IWYU pragma" annotations, such as

- Common header files such as c.h, postgres.h should be "always_keep".

- System headers included in c.h, postgres.h etc. should be considered
"export".

- Some portability headers such as getopt_long.h should be
"always_keep", so they are not considered superfluous on some
platforms.

- Certain system headers included from portability headers should be
considered "export" because the purpose of the portability header is
to wrap them.

- Superfluous includes marked as "for backward compatibility" get a
formal IWYU annotation.

- Generated header included in utils/syscache.h is marked exported.
This is a very commonly used include and this avoids lots of
complaints.

Discussion: https://www.postgresql.org/message-id/flat/9395d484-eff4-47c2-b276-8e228526c8ae(at)eisentraut(dot)org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/74938d132093bc0ad99a5420da2e99edaa3734f9

Modified Files
--------------
src/include/c.h | 5 +++++
src/include/getopt_long.h | 3 ++-
src/include/pg_getopt.h | 5 +++--
src/include/pg_trace.h | 2 +-
src/include/pgstat.h | 6 +++---
src/include/port/pg_iovec.h | 2 +-
src/include/port/pg_pthread.h | 2 +-
src/include/postgres.h | 5 +++++
src/include/postgres_ext.h | 1 +
src/include/postgres_fe.h | 5 +++++
src/include/utils/syscache.h | 2 +-
src/interfaces/libpq/libpq-int.h | 2 ++
12 files changed, 30 insertions(+), 10 deletions(-)