pgsql: Add --parallel option to vacuumdb command.

Add --parallel option to vacuumdb command.

Commit 40d964ec99 allowed vacuum command to leverage multiple CPUs by
invoking parallel workers to process indexes.  This commit provides a
'--parallel' option to specify the parallel degree used by vacuum command.

Author: Masahiko Sawada, with few modifications by me
Reviewed-by: Mahendra Singh and Amit Kapila
Discussion: https://postgr.es/m/CAD21AoDTPMgzSkV4E3SFo1CH_x50bf5PqZFQf4jmqjk-C03BWg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/47bc9ced0d0e96523e2c639c7066c9aede189ed7

Modified Files
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doc/src/sgml/ref/vacuumdb.sgml    | 18 +++++++++++++++
src/bin/scripts/t/100_vacuumdb.pl | 13 ++++++++++-
src/bin/scripts/vacuumdb.c        | 47 ++++++++++++++++++++++++++++++++++++++-
3 files changed, 76 insertions(+), 2 deletions(-)