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 <[email protected]>
Reviewed-by: Chao Li <[email protected]>
Reviewed-by: Álvaro Herrera <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Discussion: https://postgr.es/m/5F301096-921A-427D-8EC1-EBAEC2A35082@yesql.se