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

We don't have any such platforms now, but might in the future.

Also, detect cases when a tablespace symlink points to a path that
is longer than we can handle, and give a warning.

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/9c32da5caa2efc9bdc7f04ae26488211ff219e8a

Modified Files
--------------
src/backend/replication/basebackup.c |   25 ++++++++++++++++++++++---
1 files changed, 22 insertions(+), 3 deletions(-)

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Next:	From: Tom Lane	Date: 2011-12-07 18:34:33
	Subject: pgsql: Fix corner cases in readlink() usage.
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	Subject: Re: [COMMITTERS] pgsql: plpython: Add SPI cursor support