pg_settings.pending_restart not set when line removed

Hi

I got a report from Gabriele Bartolini and team that the pg_settings
view does not get the pending_restart flag set when a setting's line is
removed from a file (as opposed to its value changed).

The explanation seems to be that GUC_PENDING_RESTART is set by
set_config_option, but when ProcessConfigFileInternal is called only to
provide data (applySettings=true), then set_config_option is never
called and thus the flag doesn't get set.

I tried the attached patch, which sets GUC_PENDING_RESTART if we're
doing pg_file_settings(). Then any subsequent read of pg_settings will
have the pending_restart flag set. This seems to work correctly, and
consistently with the case where you change a line (without removing it)
in unpatched master.

You could argue that this is *weird* (why does reading pg_file_settings
set a flag in global state?) ... but that weirdness is not something
this patch is introducing.

--
Álvaro Herrera PostgreSQL Developer — https://www.EnterpriseDB.com/
"Hay dos momentos en la vida de un hombre en los que no debería
especular: cuando puede permitírselo y cuando no puede" (Mark Twain)