pgsql: Change local_preload_libraries to PGC_USERSET

Change local_preload_libraries to PGC_USERSET

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

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/584e35d17c00b69d598522d3cf321fbd9b7dbd03

Modified Files
--------------
doc/src/sgml/config.sgml | 17 ++++++++++++++---
src/backend/utils/misc/guc.c | 2 +-
2 files changed, 15 insertions(+), 4 deletions(-)