move setpgid() from libminijail to minijail0

Setting the process group is useful for minijail0, since it makes killing
minijail and its subprocesses easier. Move this functionality to minijail0,
which allows clients of libminijail to keep the child in their own process
group.

BUG=chromium:775367
TEST=`vm_launcher run` no longer gets SIGTTOU

Change-Id: I2b6b7fe59f50abc452d74e517d196c750d17453b
2 files changed