pgsql: Add sourcefile/sourceline data to EXEC_BACKEND GUC transmission

Add sourcefile/sourceline data to EXEC_BACKEND GUC transmission files.

This oversight meant that on Windows, the pg_settings view would not
display source file or line number information for values coming from
postgresql.conf, unless the backend had received a SIGHUP since starting.

In passing, also make the error detection in read_nondefault_variables a
tad more thorough, and fix it to not lose precision on float GUCs (these
changes are already in HEAD as of my previous commit).

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/99576e01a18ba3558aac20ef1715f1627ec70939

Modified Files
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src/backend/utils/misc/guc.c | 36 +++++++++++++++++++++++++++---------
1 files changed, 27 insertions(+), 9 deletions(-)