pgsql: Disallow a digit as the first character of a variable name in pg

Disallow a digit as the first character of a variable name in pgbench.

The point of this restriction is to avoid trying to substitute variables
into timestamp literal values, which may contain strings like '12:34'.

There is a good deal more that should be done to reduce pgbench's
tendency to substitute where it shouldn't. But this is sufficient to
solve the case complained of by Jaime Soler, and it's simple enough
to back-patch.

Back-patch to v11; before commit 9d36a3866, pgbench had a slightly
different definition of what a variable name is, and anyway it seems
unwise to change long-stable branches for this.

Fabien Coelho

Discussion: https://postgr.es/m/alpine.DEB.2.22.394.2006291740420.805678@pseudo

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d8bb22ab3c9982aeb2b7cb1df964c9fec56fde39

Modified Files
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doc/src/sgml/ref/pgbench.sgml | 2 +-
src/bin/pgbench/pgbench.c | 32 +++++++++++++++++++++++---------
2 files changed, 24 insertions(+), 10 deletions(-)