Add configuration flags for using Hints and Solutions with and without knowls

Remaining errors:
 1. the number of attempts is often off by 1 or 2 -- this is a very old bug
 2. the hints checkbox is often displayed prematurely.  This is related to the
    fact that Problem.pm doesn't know the number of hints to allow until after
    the problem is rendered.
 3.  We are using showHint and showHints for all sorts of things.
     There is also a showHintLimit which is supposed to count number of attempts      after which the hint shows up. Not sure ANY of these are working properly.
     The variable names should be changed to be more distinctive.
 4. The instructor hints are not displayed automatically when the page is entered
    from a nearby problem, only after the page has been submitted.  This relatd to
    issue 1. above.

Section taken from defaults.config

 .# By default hints and solutions are ALWAYS shown for professors (and above) so that they
 .# can check hints and solutions easily and make corrections.
 .# To disable this feature set the permission  levels
 .#               always_show_hint  to  "nobody" (by default this is "professor")
 .# and   always_show_solution to "nobody" (by default this is "professor")
 .# This is done in the %permissions section above.

 .# If always_show_hint is set to "nobody" then hints are shown, even to professors, only after
 .# a certain number of submissions have occurred.  This number is set in each problem with
 .# the variable $main::showHints

 .# Use knowls for hints
$pg{options}{use_knowls_for_hints}          = 1;

 .# Use knowls for solutions
$pg{options}{use_knowls_for_solutions}      = 1;

 .# The buttons below are active only if knowls are being used. If set to 1 then the hints (and solutions)
 .# checkboxes are shown and when these are checked and the problem resubmitted THEN the knowls outline
 .# appears.  I can't immediately think of a useful case where these should be set to 1.  If knowls are not being
 .# used then these checkboxes are ALWAYS shown when a hint or solution is available and the value
 .# of these two options is ignored.

 .# Show solution checkbox
$pg{options}{show_solution_checkbox} = 0;

 .# Show hint checkbox
$pg{options}{show_hint_checkbox}     = 0;