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@2ndQuadrant.fr>