pgsql: Re-allow underscore as first character of custom GUC names.

Re-allow underscore as first character of custom GUC names.

Commit 3db826bd5 intended that valid_custom_variable_name's
rules for valid identifiers match those of scan.l.  However,
I (tgl) had some kind of brain fade and put "_" in the wrong
list.

Fix by Japin Li, per bug #17415 from Daniel Polski.

Discussion: https://postgr.es/m/17415-ebdb683d7e09a51c@postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/bd74c4037c4ee268db46e983bcc0f1e0a9f7ab72

Modified Files
--------------
src/backend/utils/misc/guc.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)