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.

Fixes [Bug #6726]

Init functions are called only once, cache is useless.

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[ruby-dev:50267] [Bug #13953]

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* ext: use rb_check_arity and rb_error_arity to raise
  ArgumentError.  [Feature #9025]

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* ext/bigdecimal/bigdecimal.c (BigDecimal_new): use PRIsVALUE for
  rb_raise() and rb_warn().
* ext/openssl/ossl_cipher.c (ossl_cipher_init): ditto.
* ext/racc/cparse/cparse.c (extract_user_token): ditto.
* ext/syslog/syslog.c (mSyslog_log): ditto.

* ext/openssl/ossl.h (OSSL_Check_Kind, OSSL_Check_Instance): now
  ossl_raise() also accepts PRIsVALUE.
* ext/openssl/ossl_asn1.c (ossl_asn1_default_tag):

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  when $SAFE is set to 4.  $SAFE=4 is now obsolete.
  [ruby-core:55222] [Feature #8468]

* object.c (rb_obj_untrusted, rb_obj_untrust, rb_obj_trust):
  Kernel#untrusted?, untrust, and trust are now deprecated.
  Their behavior is same as tainted?, taint, and untaint,
  respectively.

* include/ruby/ruby.h (OBJ_UNTRUSTED, OBJ_UNTRUST): OBJ_UNTRUSTED()
  and OBJ_UNTRUST() are aliases of OBJ_TAINTED() and OBJ_TAINT(),
  respectively.

* array.c, class.c, debug.c, dir.c, encoding.c, error.c, eval.c,
  ext/curses/curses.c, ext/dbm/dbm.c, ext/dl/cfunc.c,
  ext/dl/cptr.c, ext/dl/dl.c, ext/etc/etc.c, ext/fiddle/fiddle.c,
  ext/fiddle/pointer.c, ext/gdbm/gdbm.c, ext/readline/readline.c,
  ext/sdbm/init.c, ext/socket/ancdata.c, ext/socket/basicsocket.c,
  ext/socket/socket.c, ext/socket/udpsocket.c,
  ext/stringio/stringio.c, ext/syslog/syslog.c, ext/tk/tcltklib.c,
  ext/win32ole/win32ole.c, file.c, gc.c, hash.c, io.c, iseq.c,
  load.c, marshal.c, object.c, proc.c, process.c, random.c, re.c,
  safe.c, string.c, thread.c, transcode.c, variable.c,
  vm_insnhelper.c, vm_method.c, vm_trace.c: remove code for
  $SAFE=4.

* test/dl/test_dl2.rb, test/erb/test_erb.rb,
  test/readline/test_readline.rb,
  test/readline/test_readline_history.rb, test/ruby/test_alias.rb,
  test/ruby/test_array.rb, test/ruby/test_dir.rb,
  test/ruby/test_encoding.rb, test/ruby/test_env.rb,
  test/ruby/test_eval.rb, test/ruby/test_exception.rb,
  test/ruby/test_file_exhaustive.rb, test/ruby/test_hash.rb,
  test/ruby/test_io.rb, test/ruby/test_method.rb,
  test/ruby/test_module.rb, test/ruby/test_object.rb,
  test/ruby/test_pack.rb, test/ruby/test_rand.rb,
  test/ruby/test_regexp.rb, test/ruby/test_settracefunc.rb,
  test/ruby/test_struct.rb, test/ruby/test_thread.rb,
  test/ruby/test_time.rb: remove tests for $SAFE=4.

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