timestamp fix for AIX and IRIX as discussed

Please apply the following patch to fix AIX and IRIX timestamp behavior
as previously discussed.

It makes AIX and IRIX not use DST for dates before 1970.

The following expected files need to be removed from the regression tests,
they contain wrong results and are not needed any more.

src/test/regress/expected/horology-1947-PDT.out
src/test/regress/expected/tinterval-1947-PDT.out
src/test/regress/expected/abstime-1947-PDT.out

Thanks
Andreas