If `posix_openpt` is available, also `ptsname` should be available.

Although glibc `ptsname_r` man page mentions Tru64 and HP-UX, this
function appears to be declared obsolete on both.

icc now seems to provide libutil.so that is not related to pty.
This extconf.rb wrongly finds it and adds `-lutil`, but `ruby -rpty`
fails because it cannot find libutil.so on the runtime.

http://rubyci.s3.amazonaws.com/icc-x64/ruby-master/log/20220815T210005Z.fail.html.gz
```
Exception raised:
<#<LoadError: libutil.so: cannot open shared object file: No such file or directory - /home/chkbuild/chkbuild/tmp/build/20220815T210005Z/ruby/.ext/x86_64-linux/pty.so>>
```

This change makes extconf.rb check libutil only on OpenBSD.

`nacl` support already removed by https://github.com/ruby/ruby/pull/1726.

[Fix GH-1920]

From: yuuji.yaginuma <yuuji.yaginuma@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64013 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57527 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

* ext/pty/pty.c (establishShell): honor USER environment variable
  and login name over uid, one pid can be shared by some login
  names.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54822 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

g -L frozen_string_literal ext/**/*.rb|xargs ruby -Ka -e'ARGV.each{|fn|puts
fn;open(fn,"r+"){|f|s=f.read.sub(/\A(#!.*\n)?(#.*coding.*\n)?/,"\\&#
frozen_string_literal: false\n");f.rewind;f.write s}}'

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53143 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

* ext/pty/extconf.rb: modify $INCFLAGS to include internal.h

* ext/pty/pty.c: include internal.h.
  (chfunc): don't call rb_thread_atfork_before_exec.  use
  rb_exec_async_signal_safe instead of rb_f_exec.
  (establishShell): set up earg.  use rb_fork_async_signal_safe
  instead of rb_fork_err.

* internal.h (rb_exec_async_signal_safe): declared.
  (rb_fork_async_signal_safe): declared.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36002 b2dd03c8-39d4-4d8f-98ff-823fe69b080e