pgsql: Add GUC to show EXEC_BACKEND state

Add GUC to show EXEC_BACKEND state

There is no straightforward way to determine if a cluster is running
in EXEC_BACKEND mode or not, which is useful for tests to know. This
adds a GUC debug_exec_backend similar to debug_assertions which will
be true when the server is running in EXEC_BACKEND mode.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/5F301096-921A-427D-8EC1-EBAEC2A35082@yesql.se

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b3fe098d330ff6a7c23019d66a600072b1d4664d

Modified Files
--------------
doc/src/sgml/config.sgml                  | 17 +++++++++++++++++
src/backend/utils/misc/guc_parameters.dat |  7 +++++++
src/backend/utils/misc/guc_tables.c       |  7 +++++++
3 files changed, 31 insertions(+)