Postgres get a list of all user defined trigger functions

If you want to select all functions and leave out the ones created by the system, you can add a JOIN with the table pg_user and select the functions that were not created by postgres:

SELECT p.oid,proname,prosrc,u.usename
FROM  pg_proc p  
JOIN  pg_user u ON u.usesysid = p.proowner  
WHERE usename <> 'postgres' AND prorettype = 2279;

Alternatively, you could also specify a user (or many) to the where clause within a IN expression:

WHERE usename IN ('user1','user2')

EDIT: see comments. If the sysadmin created functions using the user postgres I would suggest to change the owner to another user.

ALTER TABLE public.mytable OWNER TO myuser;

If for some reason ALTER TABLE is not possible, try selecting by language (sql, plpgsql, etc) or just filter out the functions containing the language internal:

SELECT p.oid,proname,prosrc,u.usename
FROM  pg_proc p  
JOIN  pg_user u ON u.usesysid = p.proowner  
WHERE prolang = 13436 AND prorettype = 2279;

Or maybe ..

SELECT p.oid,proname,prosrc,u.usename
FROM  pg_proc p  
JOIN  pg_user u ON u.usesysid = p.proowner  
WHERE prolang <> 12 AND prorettype = 2279; --12=internal

See table pg_language to get the oid of your function's language, e.g. of my instance:

SELECT oid, lanname FROM pg_language;

  oid  | lanname  
-------+----------
    12 | internal
    13 | c
    14 | sql
 13436 | plpgsql