hurd: implement RLIMIT_AS against Mach RPCs

hurd: implement RLIMIT_AS against Mach RPCs

Check for VM limit RPCs

  * config.h.in: add #undef for HAVE_MACH_VM_GET_SIZE_LIMIT and
    HAVE_MACH_VM_SET_SIZE_LIMIT.
  * sysdeps/mach/configure.ac: use mach_RPC_CHECK to check for
    vm_set_size_limit and vm_get_size_limit RPCs in gnumach.defs.
  * sysdeps/mach/configure: regenerate file.

Use vm_get_size_limit to initialize RLIMIT_AS

  * hurd/hurdrlimit.c(init_rlimit): use vm_get_size_limit to initialize
    RLIMIT_AS entry of the _hurd_rlimits array.

Notify the kernel of the new VM size limits

  * sysdeps/mach/hurd/setrlimit.c: use the vm_set_size_limit RPC,
    if available, to notify the kernel of the new limits. Retry RPC
    calls if they were interrupted by a signal.
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