Make contrib modules' installation scripts more secure.

 Make contrib modules' installation scripts more secure.
 
 Hostile objects located within the installation-time search_path could
 capture references in an extension's installation or upgrade script.
 If the extension is being installed with superuser privileges, this
 opens the door to privilege escalation.  While such hazards have existed
 all along, their urgency increases with the v13 "trusted extensions"
 feature, because that lets a non-superuser control the installation path
 for a superuser-privileged script.  Therefore, make a number of changes
 to make such situations more secure:
 
 * Tweak the construction of the installation-time search_path to ensure
 that references to objects in pg_catalog can't be subverted; and
 explicitly add pg_temp to the end of the path to prevent attacks using
 temporary objects.
 
 * Disable check_function_bodies within installation/upgrade scripts,
 so that any security gaps in SQL-language or PL-language function bodies
 cannot create a risk of unwanted installation-time code execution.
 
 * Adjust lookup of type input/receive functions and join estimator
 functions to complain if there are multiple candidate functions.  This
 prevents capture of references to functions whose signature is not the
 first one checked; and it's arguably more user-friendly anyway.
 
 * Modify various contrib upgrade scripts to ensure that catalog
 modification queries are executed with secure search paths.  (These
 are in-place modifications with no extension version changes, since
 it is the update process itself that is at issue, not the end result.)
 
 Extensions that depend on other extensions cannot be made fully secure
 by these methods alone; therefore, revert the "trusted" marking that
 commit eb67623c9 applied to earthdistance and hstore_plperl, pending
 some better solution to that set of issues.
 
 Also add documentation around these issues, to help extension authors
 write secure installation scripts.
 
 Patch by me, following an observation by Andres Freund; thanks
 to Noah Misch for review.
 
 Security: CVE-2020-14350