/*
* set process group (setpgid/old setpgrp)
*
* caller does setpgid(targpid, targpgid)
*
* pgid must be in valid range (EINVAL)
* pid must be caller or child of caller (ESRCH)
* if a child
* pid must be in same session (EPERM)
* pid can't have done an exec (EACCES)
* if pgid != pid
* there must exist some pid in same session having pgid (EPERM)
* pid must not be session leader (EPERM)
*
* Permission checks now in proc_enterpgrp()
*/