io + socket: make pipes and sockets nonblocking by default

All normal Ruby IO methods (IO#read, IO#gets, IO#write, ...) are
all capable of appearing to be "blocking" when presented with a
file description with the O_NONBLOCK flag set; so there is
little risk of incompatibility within Ruby-using programs.

The biggest compatibility risk is when spawning external
programs.  As a result, stdin, stdout, and stderr are now always
made blocking before exec-family calls.

This change will make an event-oriented MJIT usable if it is
waiting on pipes on POSIX_like platforms.

It is ALSO necessary to take advantage of (proposed lightweight
concurrency (aka "auto-Fiber") or any similar proposal for
network concurrency: https://bugs.ruby-lang.org/issues/13618

Named-pipe (FIFO) are NOT yet non-blocking by default since
they are rarely-used and may introduce compatibility problems
and extra syscall overhead for a common path.

Please revert this commit if there are problems and if I am afk
since I am afk a lot, lately.

[ruby-core:89950] [Bug #14968]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65922 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/socket/test_basicsocket.rb

@@ -159,8 +159,9 @@ class TestSocket_BasicSocket < Test::Unit::TestCase
       set_nb = true
       buf = String.new
       if ssock.respond_to?(:nonblock?)
-        assert_not_predicate(ssock, :nonblock?)
-        assert_not_predicate(csock, :nonblock?)
+        assert_predicate(ssock, :nonblock?)
+        assert_predicate(csock, :nonblock?)
+        csock.nonblock = ssock.nonblock = false
 
         # Linux may use MSG_DONTWAIT to avoid setting O_NONBLOCK
         if RUBY_PLATFORM.match?(/linux/) && Socket.const_defined?(:MSG_DONTWAIT)