pgsql: Use posix_fallocate() for new WAL files, where available.

Use posix_fallocate() for new WAL files, where available.

This function is more efficient than actually writing out zeroes to
the new file, per microbenchmarks by Jon Nelson. Also, it may reduce
the likelihood of WAL file fragmentation.

Jon Nelson, with review by Andres Freund, Greg Smith and me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/269e780822abb2e44189afaccd6b0ee7aefa7ddd

Modified Files
--------------
configure                         |    3 +-
configure.in                      |    2 +-
src/backend/access/transam/xlog.c |   93 +++++++++++++++++++++++--------------
src/include/pg_config.h.in        |    3 ++
src/include/pg_config.h.win32     |    3 ++
5 files changed, 67 insertions(+), 37 deletions(-)