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

spec/ruby/core/io/read_nonblock_spec.rb

@@ -44,7 +44,6 @@ describe "IO#read_nonblock" do
   platform_is_not :windows do
     it 'sets the IO in nonblock mode' do
       require 'io/nonblock'
-      @read.nonblock?.should == false
       @write.write "abc"
       @read.read_nonblock(1).should == "a"
       @read.nonblock?.should == true

spec/ruby/core/io/write_nonblock_spec.rb

@@ -76,7 +76,6 @@ describe 'IO#write_nonblock' do
   platform_is_not :windows do
     it 'sets the IO in nonblock mode' do
       require 'io/nonblock'
-      @write.nonblock?.should == false
       @write.write_nonblock('a')
       @write.nonblock?.should == true
     end

spec/ruby/library/socket/socket/connect_spec.rb

@@ -34,6 +34,12 @@ describe 'Socket#connect' do
 
       lambda {
         @client.connect(@server.getsockname)
+
+        # A second call needed if non-blocking sockets become default
+        # XXX honestly I don't expect any real code to care about this spec
+        # as it's too implementation-dependent and checking for connect()
+        # errors is futile anyways because of TOCTOU
+        @client.connect(@server.getsockname)
       }.should raise_error(Errno::EISCONN)
     end