pgsql: Adjust assorted hint messages that list all valid options.

Adjust assorted hint messages that list all valid options.

Instead of listing all valid options, we now try to provide one
that looks similar.  Since this may be useful elsewhere, this
change introduces a new set of functions that can be reused for
similar purposes.

Author: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/b1f9f399-3a1a-b554-283f-4ae7f34608e2@enterprisedb.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/5ac51c8c9e4434140f4ba45b7bdb38896b48cc64

Modified Files
--------------
contrib/dblink/dblink.c                        | 27 +++++----
contrib/dblink/expected/dblink.out             |  1 -
contrib/file_fdw/expected/file_fdw.out         |  2 -
contrib/file_fdw/file_fdw.c                    | 24 +++++---
contrib/postgres_fdw/expected/postgres_fdw.out |  3 +-
contrib/postgres_fdw/option.c                  | 23 +++++---
src/backend/foreign/foreign.c                  | 26 +++++---
src/backend/utils/adt/varlena.c                | 82 ++++++++++++++++++++++++++
src/include/utils/varlena.h                    | 12 ++++
src/test/regress/expected/foreign_data.out     |  6 +-
10 files changed, 159 insertions(+), 47 deletions(-)