sysvshmem on GNU/Hurd

[Roland McGrath]

Rename those internal entrypoints to _hurd_foo like all the others.
Use "hurd" in the header file name, like hurdsysvshm.h if it's private.
The header needs some more comments.  If it's for talking to a particular
server it should say something about that.  I am inclined to say that the
magic file name details should be in <hurd/paths.h> macros.

Send the bits/stat.h changes separately.  Is S_IMMAP0 already used by Hurd
server code in the trunk?