pgsql: Re-allow custom GUC names that have more than two components.

Re-allow custom GUC names that have more than two components.

Commit 3db826bd5 disallowed this case, but it turns out that some
people are depending on it.  Since the core grammar has allowed
it since 3dc37cd8d, it seems like this code should fall in line.

Per bug #17045 from Robert Sosinski.

Discussion: https://postgr.es/m/17045-6a4a9f0d1513f72b@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2955c2be79b35fa369c83fa3b5f44661cb88afa9

Modified Files
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src/backend/utils/misc/guc.c      | 15 ++++++++-------
src/test/regress/expected/guc.out | 20 ++++++++++++++++++--
src/test/regress/sql/guc.sql      |  5 +++++
3 files changed, 31 insertions(+), 9 deletions(-)