[Bug #21448] Use `getentropy(2)` only on macOS

	If this is not a system call, then it is using getrandom (which would
	have been tried already), and cannot be used as a replacement for the
	random devices.

	Restore getrandom(2) path for Linux with glibc 2.36 or later

	This is a follow-up to commit b120f5e38d9c (avoid fork-unsafe arc4random
	implementations, 2018-09-04).

	Avoid defining a no-op fill_random_bytes_syscall() if arc4random_buf(3)
	exists, but we are unsure if it is fork-safe. Check for other options
	instead. IOW, see if getrandom(2) is available.

	glibc 2.36, released in 2022, started to provide arc4random_buf(3) on
	Linux. This causes fill_random_bytes_syscall() to use neither of them
	and makes Random.urandom solely rely on getentropy(3) via
	fill_random_bytes_urandom().

	While the glibc implementation is safe, I did not add it to the list
	because using getrandom(2) directly is preferable on Linux.

	[Bug #21448] Reorder trials in `fill_random_bytes`

	First try dedicated system calls, such as `getrandom` or `getentropy`,
	next possible libraries, then fallback to `/dev/urandom`.

when the RUBY_FREE_ON_SHUTDOWN environment variable is set, manually free memory at shutdown.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>
Co-authored-by: Peter Zhu <peter@peterzhu.ca>

* Make its usage more obvious (by providing more detailed
  examples);
* Include mention of it into Random itself;
* Small cleanups of docs.

Distinguish initialization by single word from initialization by
array.

[Misc #18891]