Create new pgroup when spawning process in chdir

[davishmcclurg]

Process.spawn is leaving orphaned processes when run inside
Dir.chdir:

>> JRUBY_VERSION
=> "9.2.0.0"
>> Dir.chdir('/tmp') { Process.spawn('cat') }
=> 88255

$ ps xao pid,ppid,pgid,comm | grep -E ' (cat|sh)'
88255 88092 88092 sh
88256 88255 88092 cat

>> Process.kill('TERM', 88255)
=> 1

$ ps xao pid,ppid,pgid,comm | grep -E ' (cat|sh)'
88256     1 88092 cat

I believe the issue is that adding the cd command (for chdir) causes a
second process to start. That process is grouped with the main java one
and doesn't die when its parent (sh) is killed.

This change creates a new process group for the sh command, which
causes the child process to die when the parent is killed.

I ran into this issue using Foreman to manage some JRuby processes.

[davishmcclurg]

Another option for this is using "cd '" + eargp.chdir_dir + "'; exec " to have the process be replaced after changing directories. I wasn't sure what that would mean for Windows support, though, and I don't have access to a machine for testing.

[headius]

This looks good, and it's a great discovery. Thank you!