pgsql: Various improvements of skipping index scan during vacuum techni

Various improvements of skipping index scan during vacuum technics

- Change vacuum_cleanup_index_scale_factor GUC to PGC_USERSET.
vacuum_cleanup_index_scale_factor GUC was defined as PGC_SIGHUP. But this
GUC affects not only autovacuum. So it might be useful to change it from user
session in order to influence manually runned VACUUM.
- Add missing tab-complete support for vacuum_cleanup_index_scale_factor
reloption.
- Fix condition for B-tree index cleanup.
Zero value of vacuum_cleanup_index_scale_factor means that user wants B-tree
index cleanup to be never skipped.
- Documentation and comment improvements

Authors: Justin Pryzby, Alexander Korotkov, Liudmila Mantrova
Reviewed by: all authors and Robert Haas
Discussion: https://www.postgresql.org/message-id/flat/20180502023025.GD7631%40telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8e12f4a250d250a89153da2eb9b91c31bb80c483

Modified Files
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doc/src/sgml/config.sgml | 35 +++++++++++++++++++++++++++--------
src/backend/access/nbtree/nbtpage.c | 2 +-
src/backend/access/nbtree/nbtree.c | 19 ++++++++++---------
src/backend/utils/misc/guc.c | 2 +-
src/bin/psql/tab-complete.c | 6 ++++--
5 files changed, 43 insertions(+), 21 deletions(-)