PG-1361 Fix relocatablility

[jeltz]

Creating the extension in another schema was broken.

$ CREATE EXTENSION pg_tde SCHEMA foo;
CREATE EXTENSION
# SELECT foo.pg_tde_add_key_provider_file('a', '""');
ERROR:  function pg_tde_add_key_provider(unknown, character varying, json) does not exist
LINE 4:     SELECT pg_tde_add_key_provider('file', provider_name,
                   ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
QUERY:  
    -- JSON keys in the options must be matched to the keys in
    -- load_file_keyring_provider_options function.
    SELECT pg_tde_add_key_provider('file', provider_name,
                json_object('type' VALUE 'file', 'path' VALUE COALESCE(file_path, '')));

This PR fixes the issue by using BEGIN ATOMIC for SQL functions and setting the search_path for plpgsql functions. But since the plpgsql functions are not reloctable we set relocatable = false in the extension control file. If we want to make the extension relocatable I propose we do it as a separate PR.

I also did some style fixes which you are free to object to. They are as a separate commit so that can be dropped or changed if you do not like it.

[dutow]

Can you also add a testcase for it, or modify on of the existing SQL tests to create the extension in a different schema?

[jeltz]

Can you also add a testcase for it, or modify on of the existing SQL tests to create the extension in a different schema?

Sure, will add one!

[jeltz]

@dutow A test has been added.