pgsql: Add 'unit' parameter to ExplainProperty{Integer,Float}.

Add 'unit' parameter to ExplainProperty{Integer,Float}.

This allows to deduplicate some existing code, but mainly avoids some
duplication in upcoming commits.

In passing, fix variable names indicating wrong unit (seconds instead
of ms).

Author: Andres Freund
Discussion: https://postgr.es/m/20180314002740.cah3mdsonz5mxney@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7a50bb690b4837d29e715293c156cff2fc72885c

Modified Files
--------------
contrib/file_fdw/file_fdw.c    |   2 +-
src/backend/commands/explain.c | 199 ++++++++++++++++++++++-------------------
src/include/commands/explain.h |   8 +-
3 files changed, 113 insertions(+), 96 deletions(-)