pgsql: Add ALTER EXTENSION ADD/DROP ACCESS METHOD, and use it in pg_upg

Add ALTER EXTENSION ADD/DROP ACCESS METHOD, and use it in pg_upgrade.

Without this, an extension containing an access method is not properly
dumped/restored during pg_upgrade --- the AM ends up not being a member
of the extension after upgrading.

Another oversight in commit 473b93287, reported by Andrew Dunstan.

Report: <f7ac29f3-515c-2a44-21c5-ec925053265f@dunslane.net>

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/f40334b85cf89f1eafd04d31665704fd173e317c

Modified Files
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doc/src/sgml/ref/alter_extension.sgml |  1 +
src/backend/parser/gram.y             | 11 ++++++++++-
src/bin/pg_dump/pg_dump.c             |  3 +++
3 files changed, 14 insertions(+), 1 deletion(-)