* io.c (io_read_nonblock): support non-blocking reads without raising
  exceptions. As in: `io.read_nonblock(size, exception: false)`
  [ruby-core:38666] [Feature #5138]
* ext/openssl/ossl_ssl.c (ossl_ssl_read_internal): ditto
* ext/stringio/stringio.c (strio_sysread): ditto
* io.c (rb_io_write_nonblock): support non-blocking writes without
  raising an exception.
* ext/openssl/ossl_ssl.c (ossl_ssl_write_internal): ditto
* test/openssl/test_pair.rb (class OpenSSL): tests
* test/ruby/test_io.rb (class TestIO): ditto
* test/socket/test_nonblock.rb (class TestSocketNonblock): ditto
* test/stringio/test_stringio.rb (class TestStringIO): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42695 b2dd03c8-39d4-4d8f-98ff-823fe69b080e