This reworks PTY#spawn to use openpty(3) (available on BSD, OS X and Linux) and Process#spawn instead of forkpty(3), which as the name suggests, forks and is thus unsafe. It also fixes use with multiple arguments.

It also adds PTY#open and PTY#check, and properly defines PTY#getpty as an alias to PTY#spawn.

It passes MRI's test_pty.rb, after a slight tweak to close a race condition:

Finished tests in 15.752000s, 0.9523 tests/s, 2.4759 assertions/s.
15 tests, 39 assertions, 0 failures, 0 errors, 0 skips

ruby -v: jruby 9.0.1.0-SNAPSHOT (2.2.2) 2015-08-01 6a9fcf8 OpenJDK 64-Bit Server VM 25.51-b03 on 1.8.0_51-b16 +jit [FreeBSD-amd64]

Needs testing and review, especially outside FreeBSD.