Disallow dollar sign in operator names, instead allow it as a non-first

Disallow dollar sign in operator names, instead allow it as a non-first
character in identifiers.  The first change eliminates the current need
to put spaces around parameter references, as in "x<=$2".  The second
change improves compatibility with Oracle and some other RDBMSes.  This
was discussed and agreed to back in January, but did not get done.