libpq connection options as returned by PQconndefaults() have a
"dispchar" field that determines (among other things) whether an option
is a "debug" option, which shouldn't be shown by default to clients.
postgres_fdw makes use of that to control which connection options to
accept from a foreign server configuration.

Curiously, the "options" option, which allows passing configuration
settings to the backend server, was listed as a debug option, which
prevented it from being used by postgres_fdw.  Maybe it was once meant
for debugging, but it's clearly in general use nowadays.

So change the dispchar for it to be the normal non-debug case.  Also
remove the "debug" reference from its label field.
Reported-by: Shinoda, Noriyoshi <noriyoshi.shinoda@hpe.com>