Fix corner cases in readlink() usage.

Make sure all calls are protected by HAVE_READLINK, and get the buffer
overflow tests right.  Be a bit more paranoid about string length in
_tarWriteHeader(), too.

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/85d85ff7ef04510f24fe5897e5535d765d7a293c

Modified Files
--------------
src/backend/replication/basebackup.c |   52 ++++++++++++++++++++++++---------
1 files changed, 38 insertions(+), 14 deletions(-)

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	Subject: pgsql: Avoid using readlink() on platforms that don't support it