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

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
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REL9_1_STABLE

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

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