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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master

Details
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http://git.postgresql.org/pg/commitdiff/1f422db663aa291be4af4b079e7d1b0ef0d78af2

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