SHM_REMAP was incorrectly defined to 030000 which collides with
SHM_RDONLY and SHM_RND. Renumber to 040000 (incidentally matching
Linux).
This is an ABI break, but the previous ABI was unusable (SHM_REMAP would
imply SHM_RDONLY and vice versa). Fortunately SHM_REMAP has very few
consumers in the wild (I spotted openjdk for Linux, libfabric, MIPCH,
and one other MPI thing in Debian code search)
Fixes: ea7e7006db10 Implement shmat(2) flag SHM_REMAP.