pgsql: Be more conservative about removing tablespace "symlinks".

Be more conservative about removing tablespace "symlinks".

Don't apply rmtree(), which will gleefully remove an entire subtree,
and don't even apply unlink() unless it's symlink or a directory,
the only things that we expect to find.

Amit Kapila, with minor tweaks by me, per extensive discussions
involving Andrew Dunstan, Fujii Masao, and Heikki Linnakangas,
at least some of whom also reviewed the code.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8f15f74a44f68f9cb3a644786d3c732a5eeb237a

Modified Files
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src/backend/access/transam/xlog.c | 24 ++--------
src/backend/commands/tablespace.c | 89 +++++++++++++++++++++++++++----------
src/include/commands/tablespace.h | 1 +
3 files changed, 69 insertions(+), 45 deletions(-)