Commit 3db826bd 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