When available, use posix_fallocate() rather than ftruncate() to allocate

When available, use posix_fallocate() rather than ftruncate() to allocate
space for mmap()ed -shm files, since posix_fallocate() gives an error if
no disk space is available whereas ftruncate() is silent and leaves the system
vulnerable to a SIGBUS upon first write to the mmap()ed region.
Ticket [5eaa61ea1881040b17449ca043b6f8fd9ca55dc3]

FossilOrigin-Name: 356259617cfad04492a02912fdf781f54a2b4494