pgsql: In AtEOXact_Files, complain if any files remain unclosed at comm

In AtEOXact_Files, complain if any files remain unclosed at commit.

This change makes this module act more like most of our other low-level
resource management modules. It's a caller error if something is not
explicitly closed by the end of a successful transaction, so issue
a WARNING about it. This would not actually have caught the file leak
bug fixed in commit 231bcd080, because that was in a transaction-abort
path; but it still seems like a good, and pretty cheap, cross-check.

Discussion: https://postgr.es/m/152056616579.4966.583293218357089052@wrigleys.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9cb7db3f0c1f554cdcbbd97f78a119a19756e6ef

Modified Files
--------------
src/backend/access/transam/xact.c | 6 +++---
src/backend/postmaster/autovacuum.c | 2 +-
src/backend/postmaster/bgwriter.c | 2 +-
src/backend/postmaster/checkpointer.c | 2 +-
src/backend/postmaster/walwriter.c | 2 +-
src/backend/storage/file/fd.c | 32 +++++++++++++++++++++-----------
src/include/storage/fd.h | 2 +-
7 files changed, 29 insertions(+), 19 deletions(-)