pgsql: Use camel case for "DateStyle" in some error messages

Use camel case for "DateStyle" in some error messages

This GUC is written as camel-case in most of the documentation and the
GUC table (but not postgresql.conf.sample), and two error messages
hardcoded it with lower case characters.  Let's use a style more
consistent.

Most of the noise comes from the regression tests, updated to reflect
the GUC name in these error messages.

Author: Peter Smith
Reviewed-by: Peter Eisentraut, Álvaro Herrera
Discussion: https://postgr.es/m/CAHut+Pv-kSN8SkxSdoHano_wPubqcg5789ejhCDZAcLFceBR-w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2e7c4abe5a88a1528236a13c9ef2e351a97fa022

Modified Files
--------------
src/backend/commands/variable.c        |  2 +-
src/backend/utils/adt/datetime.c       |  2 +-
src/test/regress/expected/date.out     | 58 +++++++++++++++++-----------------
src/test/regress/expected/horology.out |  2 +-
4 files changed, 32 insertions(+), 32 deletions(-)