pgsql: Fix handling of symlinked pg_stat_tmp and pg_replslot

Fix handling of symlinked pg_stat_tmp and pg_replslot

This was already fixed in HEAD as part of 6ad8ac60 but was not
backpatched.

Also change the way pg_xlog is handled to be the same as the other
directories.

Patch from me with pg_xlog addition from Michael Paquier, test updates
from David Steele.

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b6a323a8c9b8f75a974e44cc6bb91f0d08e19e7f

Modified Files
--------------
src/backend/replication/basebackup.c         | 48 +++++++++++++++++-----------
src/bin/pg_basebackup/t/010_pg_basebackup.pl | 16 ++++++++--
2 files changed, 44 insertions(+), 20 deletions(-)