fix visibility of SecureRandom.gen_random

	Aliasing a method preserves its visibility.  These aliases turn
	formerly-public methods into private.  Should make them public
	again.  [Bug #15847]

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* lib/securerandom.rb (Random::Formatter#choose): [DOC] fix an
  example, `n` is not optional.

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[ruby-core:68098] [Feature #10849] proposed by Andrew Butterfield.

SecureRandom.choose and SecureRandom.graph is not included.
(The implementation has SecureRandom.choose but it is private.)

I feel the method name, SecureRandom.choose, doesn't represent
the behavior well.

The actual use cases of SecureRandom.graph is not obvious.



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`SecureRandom#gen_random` determines whether urandom is available or not
by trying `Random.urandom(n)`.  But, when n = 0, `Random.urandom(0)`
always succeeds even if urandom is not available, which leads to a wrong
decision.

When failed, `Random.urandom` returns nil instead of returning a shorter
string than required.  So the check for `ret.length != n` is not needed.

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  [Misc #13870][ruby-core:82654] Patch by @aycabta.

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SecureRandom.gen_random_openssl still refers to Random.raw_seed, which
is renamed to Random.urandom by r57384.  [Bug #9569]

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	* random.c (InitVM_Random): rename Random.raw_seed to
	  Random.urandom.  A quick search seems there are no practical use
	  of this method than securerandom.rb so I think it's OK to rename
	  but if there are users of it, this hunk is subject to revert.

	* test/ruby/test_rand.rb (TestRand#test_urandom): test for it.

	* lib/securerandom.rb (SecureRandom.gen_random): Prefer OS-
	  provided CSPRNG if available. Otherwise falls back to OpenSSL.
	  Current preference is:

	  1. CSPRNG routine that the OS has; one of
	     - getrandom(2),
	     - arc4random(3), or
	     - CryptGenRandom()
	  2. /dev/urandom device
	  3. OpenSSL's RAND_bytes(3)

	  If none of above random number generators are available, you
	  cannot use this module.  An exception is raised that case.



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* random.c (obj_random_bytes): base on bytes method instead of
  rand method, not to call toplevel rand method.

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  On Windows OpenSSL RAND_bytes (underlying implementation is
  RAND_poll in crypto/rand/rand_win.c) may be blocked at
  NetStatisticsGet.
  https://wiki.openssl.org/index.php/Random_Numbers#Windows_Issues
  Instead of this, use Random.raw_seed directory (whose implementation
  CryptGenRandom is one of the source of
  entropy of RAND_poll on Windows).
  https://wiki.openssl.org/index.php/Random_Numbers
  Note: CryptGenRandom function is PRNG and doesn't check its entropy,
  so it won't block. [Bug #12139]
  https://msdn.microsoft.com/ja-jp/library/windows/desktop/aa379942.aspx
  https://tools.ietf.org/html/rfc4086#section-7.1.3
  https://eprint.iacr.org/2007/419.pdf
  http://www.cs.huji.ac.il/~dolev/pubs/thesis/msc-thesis-leo.pdf

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