Improve tab completion for COPY

Hi,

I noticed that REJECT_LIMIT, an option available for COPY FROM, is not
currently supported in psql's tab completion.

Additionally, some options are only valid for COPY FROM or COPY TO, i.e.
FREEZE, ON_ERROR, FORCE_QUOTE, but psql currently suggests them for both
COPY FROM and COPY TO.
As the number of COPY options continues to grow, I feel that having
irrelevant suggestions makes tab completion noisier.

Attached patch splits the tab completion rules between COPY FROM and
COPY TO, so that only the appropriate options are suggested for each.

What do you think?

Regards,

--
Atsushi Torikoshi
Seconded from NTT DATA GROUP CORPORATION to SRA OSS K.K.