Add flags argument to dsm_create.

Add flags argument to dsm_create.

Right now, there's only one flag, DSM_CREATE_NULL_IF_MAXSEGMENTS,
which suppresses the error that would normally be thrown when the
maximum number of segments already exists, instead returning NULL.
It might be useful to add more flags in the future, such as one to
ignore allocation errors, but I haven't done that here.