handle umask

[jti-lanl]

fuse_chmod(), fuse_open() and fuse_mknod() could get umask as fuse_get_context()->umask. They could then pass this as an arg to their libmarfs counterparts. Then pftool's MARFS_Path should do something similar, using umask().

[NOTE: umask() sets the umask to the argument and returns the old value. So I guess one must call it twice, to retrieve the current value without changing it, leaving it momentarily wrong, during the period between the two calls.]