SQL> exec DBMS_SYSTEM.SET_INT_PARAM_IN_SESSION( -
>    23, 2, 'sort_area_size', 64*1024*1024);

PL/SQL procedure successfully completed.

SQL> exec DBMS_SYSTEM.SET_BOOL_PARAM_IN_SESSION( -
> 23, 2, 'sql_trace', FALSE);

PL/SQL procedure successfully completed.

SQL> exec DBMS_SYSTEM.SET_CHAR_PARAM_IN_SESSION( -
>    23, 2, 'tracefile_identifier', '123');
                  *
ERROR at line 1:
ORA-06550: line 1, column 19:
PLS-00302: component 'SET_CHAR_PARAM_IN_SESSION' must be declared
ORA-06550: line 1, column 7:
PL/SQL: Statement ignored

SQL> create or replace trigger trace_trigger_on 
   AFTER LOGON ON DATABASE
   declare
   stmt varchar2(100);
   hname varchar2(20);
   uname varchar2(20);
   begin
   select sys_context('USERENV','HOST'),sys_context('USERENV','SESSION_USER') into hname,uname from dual;
  2     stmt := 'alter session set tracefile_identifier='||hname||'_'||uname; 
  3     EXECUTE IMMEDIATE stmt;  
  4     EXECUTE IMMEDIATE 'alter session set sql_trace=true';
  end;  5    6    7    8    9   10   11   12  
 13  /

Trigger created.

SQL> connect scott/tiger
Connected.
SQL> select * from dual;

D
-
X

SQL> disconnect

the resulting trace file looks like 
-rw-r-----   1 oracle   dba         7919 Jan 27 11:57 dbatest_ora_6948_SAMGDEAB06_SCOTT.trc