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@eisentraut.org

src/include/postgres_ext.h

@@ -19,6 +19,7 @@
  *
  *-------------------------------------------------------------------------
  */
+/* IWYU pragma: always_keep */
 
 #ifndef POSTGRES_EXT_H
 #define POSTGRES_EXT_H