Fix broken definition for function name in pgbench's exprscan.l.

Fix broken definition for function name in pgbench's exprscan.l.

As written, this would accept e.g. 123e9 as a function name.  Aside
from being mildly astonishing, that would come back to haunt us if
we ever try to add float constants to the expression syntax.  Insist
that function names start with letters (or at least non-digits).

In passing reset yyline as well as yycol when starting a new expression.
This variable is useless since it's used nowhere, but if we're going
to have it we should have it act sanely.