pgsql: Allow IMPORT FOREIGN SCHEMA within pl/pgsql.

Allow IMPORT FOREIGN SCHEMA within pl/pgsql.

Since IMPORT FOREIGN SCHEMA has an INTO clause, pl/pgsql needs to be
aware of that and avoid capturing the INTO as an INTO-variables clause.
This isn't hard, though it's annoying to have to make IMPORT a plpgsql
keyword just for this. (Fortunately, we have the infrastructure now
to make it an unreserved keyword, so at least this shouldn't break any
existing pl/pgsql code.)

Per report from Merlin Moncure. Back-patch to 9.5 where IMPORT FOREIGN
SCHEMA was introduced.

Report: <CAHyXU0wpHf2bbtKGL1gtUEFATCY86r=VKxfcACVcTMQ70mCyig(at)mail(dot)gmail(dot)com>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/baebab3ace480477f210dadc4633d8d119dfa978

Modified Files
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src/pl/plpgsql/src/pl_gram.y | 47 +++++++++++++++++++++++++++++++++--------
src/pl/plpgsql/src/pl_scanner.c | 1 +
2 files changed, 39 insertions(+), 9 deletions(-)