to_timestamp overloaded to convert from Unix epoch

Please find attached a patch (diff -c against cvs HEAD) to add a
function that accepts a double precision argument assumed to be a Unix
epoch timestamp and returns timestamp with time zone, and accompanying
documentation.

Usage:

test=# select to_timestamp(200120400);
       to_timestamp
------------------------
  1976-05-05 14:00:00+09
(1 row)

If regression tests are required, I will produce some. I'd appreciate
any pointers as to what to look for, as they would be my first attempt
at writing regression tests.

Regards

Michael Glaesemann
grzm myrealbox com