This allows it to be used with ALTER ROLE SET.

Although the old setting of PGC_BACKEND prevented changes after session
start, after discussion it was more useful to allow ALTER ROLE SET
instead and just document that changes during a session have no effect.
This is similar to how session_preload_libraries works already.

An alternative would be to change things to allow PGC_BACKEND and
PGC_SU_BACKEND settings to be changed by ALTER ROLE SET.  But that might
need further research (e.g., log_connections would probably not work).

based on patch by Kyotaro Horiguchi