Questions about my ifnull function

Having a requirement to change null into a certain value in a query I
created a couple versions of an ifnull function as follows:

create or replace function "ifnull" (text, text) returns text as '
begin
  if $1 is null
  then
    return $2;
  else
    return $1;
  end if;
end;' language 'plpgsql';

create or replace function "ifnull2" (text, text) returns text as '
select case when $1 is null then $2 else $1 end;
' language 'sql';

The functions work fine but I have some questions:

1. Did I overlook a better builtin function?

2. Is there a good reason to prefer one over the other (ifnull2 seems
marginally faster)?

3. I had planned to overload the function to work with other datatypes
- ifnull(int, int) etc. but found that although my functions specify
text they seem to work correctly with some other data types (like int
and numeric) but not with others (inet) as shown below. Why isn't an
error generated when the wrong data types are passed? Examples:

steve=# select ifnull(null,'foo');
 ifnull
--------
 foo

steve=# select ifnull(null,5::int);
 ifnull
--------
 5


steve=# select ifnull(3::int, 'foo');
 ifnull
--------
 3

steve=# select ifnull(null,'10.0.0.1'::inet);
ERROR:  Function ifnull("unknown", inet) does not exist
        Unable to identify a function that satisfies the given
argument types
        You may need to add explicit typecasts

Cheers,
Steve