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