pgsql: Restructure libpq code to remove some duplicity

Restructure libpq code to remove some duplicity

There was some duplicate code to run SHOW transaction_read_only to
determine whether the server is read-write or read-only.  Reduce it by
adding another state to the state machine.

Author: Hari Babu Kommi
Reviewed-by: Takayuki Tsunakawa, Álvaro Herrera
Discussion: https://postgr.es/m/CAJrrPGe_qgdbbN+yBgEVpd+YLHXXjTruzk6RmTMhqrFig+32ag@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b438e7e7a1c58e0c20b5f46e73cbd713e8033c69

Modified Files
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src/interfaces/libpq/fe-connect.c | 85 ++++++++++++++-------------------------
src/interfaces/libpq/libpq-fe.h   |  3 +-
2 files changed, 33 insertions(+), 55 deletions(-)