pgsql: Move private declarations shared between guc.c and guc-file.l to

Move private declarations shared between guc.c and guc-file.l to new header

Further preparatory refactoring for compiling guc-file.c standalone.

Reviewed by Andres Freund
Discussion: https://www.postgresql.org/message-id/20220810171935.7k5zgnjwqzalzmtm%40awork3.anarazel.de
Discussion: https://www.postgresql.org/message-id/CAFBsxsF8Gc2StS3haXofshHCzqNMRXiSxvQEYGwnFsTmsdwNeg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/80e8450a744b1f6fa75663f37f1db3388995dc67

Modified Files
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src/backend/utils/misc/guc-file.l     |  2 +-
src/backend/utils/misc/guc.c          |  1 +
src/backend/utils/misc/guc_internal.h | 26 ++++++++++++++++++++++++++
src/include/utils/guc.h               | 10 ----------
4 files changed, 28 insertions(+), 11 deletions(-)