Warn on access/modify of $SAFE, and remove effects of modifying $SAFE

This removes the security features added by $SAFE = 1, and warns for access
or modification of $SAFE from Ruby-level, as well as warning when calling
all public C functions related to $SAFE.

This modifies some internal functions that took a safe level argument
to no longer take the argument.

rb_require_safe now warns, rb_require_string has been added as a
version that takes a VALUE and does not warn.

One public C function that still takes a safe level argument and that
this doesn't warn for is rb_eval_cmd.  We may want to consider
adding an alternative method that does not take a safe level argument,
and warn for rb_eval_cmd.

test/net/imap/test_imap_response_parser.rb

@@ -20,19 +20,6 @@ class IMAPResponseParserTest < Test::Unit::TestCase
     end
   end
 
-  def test_flag_list_safe
-    parser = Net::IMAP::ResponseParser.new
-    response = lambda {
-      $SAFE = 1
-      parser.parse(<<EOF.gsub(/\n/, "\r\n").taint)
-* LIST (\\HasChildren) "." "INBOX"
-EOF
-    }.call
-    assert_equal [:Haschildren], response.data.attr
-  ensure
-    $SAFE = 0
-  end
-
   def test_flag_list_too_many_flags
     parser = Net::IMAP::ResponseParser.new
     assert_nothing_raised do