pgsql: Add session_preload_libraries configuration parameter

Add session_preload_libraries configuration parameter

This is like shared_preload_libraries except that it takes effect at
backend start and can be changed without a full postmaster restart. It
is like local_preload_libraries except that it is still only settable by
a superuser. This can be a better way to load modules such as
auto_explain.

Since there are now three preload parameters, regroup the documentation
a bit. Put all parameters into one section, explain common
functionality only once, update the descriptions to reflect current and
future realities.

Reviewed-by: Dimitri Fontaine <dimitri(at)2ndQuadrant(dot)fr>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/070518ddab2c94afea119f2b1944c05d16792b07

Modified Files
--------------
doc/src/sgml/auto-explain.sgml | 5 +-
doc/src/sgml/config.sgml | 281 ++++++++++++++++++++++---------------
src/backend/tcop/postgres.c | 2 +-
src/backend/utils/init/miscinit.c | 6 +-
src/backend/utils/misc/guc.c | 19 ++-
src/include/miscadmin.h | 3 +-
src/include/utils/guc_tables.h | 1 +
7 files changed, 193 insertions(+), 124 deletions(-)