Move plugin util modules to compat

[cosmo0920]

plugin/*_util.rb should be marked as deprecated module.

[tagomoris]

LGTM.

[tagomoris]

CI are failing with following error: NameError: uninitialized constant Fluent::Plugin::SyslogInput::SocketUtil
@cosmo0920 Could you check it? It's a bit curious that it occurs only on Travis's osx environment, but should be fixed.

[cosmo0920]

Hmm..., It's curious.
I cannot reproduce this error in my local environment. :'(

[tagomoris]

@cosmo0920 I confirmed that this test fails on my OSX laptop when I just run test_in_syslog.rb.
Could you confirm it?

bundle exec rake test TEST=test/plugin/test_in_syslog.rb

[cosmo0920]

I've rebased and fixed errors.

[cosmo0920]

I've fixed a conflict.

[tagomoris]

Great fix. Thank you!