hurd: Implement MAP_EXCL

MAP_FIXED is defined to silently replace any existing mappings at the
address range being mapped over. This, however, is a dangerous, and only
rarely desired behavior.

Various Unix systems provide replacements or additions to MAP_FIXED:

* SerenityOS and Linux provide MAP_FIXED_NOREPLACE. If the address space
  already contains a mapping in the requested range, Linux returns
  EEXIST. SerenityOS returns ENOMEM, however that is a bug, as the
  MAP_FIXED_NOREPLACE implementation is intended to be compatible with
  Linux.

* FreeBSD provides the MAP_EXCL flag that has to be used in combination
  with MAP_FIXED. It returns EINVAL if the requested range already
  contains existing mappings. This is directly analogous to the O_EXCL
  flag in the open () call.

* DragonFly BSD, NetBSD, and OpenBSD provide MAP_TRYFIXED, but with
  different semantics. DragonFly BSD returns ENOMEM if the requested
  range already contains existing mappings. NetBSD does not return an
  error, but instead creates the mapping at a different address if the
  requested range contains mappings. OpenBSD behaves the same, but also
  notes that this is the default behavior even without MAP_TRYFIXED
  (which is the case on the Hurd too).

Since the Hurd leans closer to the BSD side, add MAP_EXCL as the primary
API to request the behavior of not replacing existing mappings. Declare
MAP_FIXED_NOREPLACE and MAP_TRYFIXED as aliases of (MAP_FIXED|MAP_EXCL),
so any existing software that checks for either of those macros will
pick them up automatically. For compatibility with Linux, return EEXIST
if a mapping already exists.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230625231751.404120-5-bugaevc@gmail.com>