private area called PRDA that remains unshared. We implement this by using
different vmspace for each share group member, and keeping the memory
appings in sync on each mmap/munmap/mprotect/break...
We use irix_saddr_sync_vmcmd and irix_saddr_sync_syscall to apply a
vmcmd or a syscall to all share group member, this makes the job a bit
easier.
Also implements {get|set}rlimit{64}.