pk: Don't pass -obig_writes to FUSE if the library doesn't support it
pk: Don't pass -obig_writes to FUSE if the library doesn't support it

- Older libfuse doesn't support -obig_writes.  Assume that if it doesn't
support the option, it won't define FUSE_CAP_BIG_WRITES either, and use
the latter to gate use of the former.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr