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 Polsk

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 Polsk