pgsql: Revert "Secure Unix-domain sockets of "make check" temporary clu

Revert "Secure Unix-domain sockets of "make check" temporary clusters."

About half of the buildfarm members use too-long directory names,
strongly suggesting that this approach is a dead end.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/8f5578d0f9681ef81bc71a3762a191d66a29c8b1

Modified Files
--------------
contrib/pg_upgrade/test.sh    |    9 ++++-----
doc/src/sgml/regress.sgml     |   34 +++++++++++++++-------------------
src/test/regress/pg_regress.c |   31 +++++++------------------------
3 files changed, 26 insertions(+), 48 deletions(-)