[PATCH v3 2/2] Add header matching mode to "COPY FROM"

COPY FROM supports the HEADER option to silently discard the header from
a CSV or text file. It is possible to load by mistake a file that
matches the expected format, for example if two text columns have been
swapped, resulting in garbage in the database.

This option adds the possibility to actually check the header to make
sure it matches what is expected and exit immediatly if it does not.

Discussion: https://www.postgresql.org/message-id/flat/CAF1-J-0PtCWMeLtswwGV2M70U26n4g33gpe1rcKQqe6wVQDrFA(at)mail(dot)gmail(dot)com
---
contrib/file_fdw/input/file_fdw.source | 6 ++
contrib/file_fdw/output/file_fdw.source | 9 ++-
doc/src/sgml/ref/copy.sgml | 8 ++-
src/backend/commands/copy.c | 84 +++++++++++++++++++++++--
src/test/regress/input/copy.source | 25 ++++++++
src/test/regress/output/copy.source | 17 +++++
6 files changed, 140 insertions(+), 9 deletions(-)