* Diagnostic directives: $$warning and $$error

The $$warning directive emits warning message.  It does not alter exit
status in any way.

The $$error directive reports a fatal error and sets exit status to
65 (or any other, if supplied as argument).

After both directives, processing is resumed at the next line.

* $$exit

New directive $$exit causes immediate termination of the program.
Decimal exit code may be supplied as argument.

* New directive: $$eval

The text between $$eval and $$end is expanded and the resulting
expansion is scanned again, producing the actual output.  This makes
it possible to create variable names on the fly and obtain their values.
Useful in loops, e.g.:

  $$loop I 0 1 2 3 4 5 6 7
  $$  eval
  \$\$ ifset VAR_$I
  Expand \$VAR_$I;
  \$\$ endif
  $$ end
  $$end

* Bugfixes

** Fix closing the $$range loop.