a select command invalid selection variable name is now a fatal error in posix mode; many test suite additions

tests/comsub-eof0.sub

@@ -1,3 +1,9 @@
+# this works as it should, but with a warning
+foo=$(cat <<EOF
+hi
+EOF)
+echo $foo
+unset -v foo
 # it's only the space before the paren that makes this an error
 # when I fix it, it will show up here
 foo=$(cat <<EOF