Support FUSE_LSEEK, for seeking for and around holes

[tv42]

Enabled by FUSE protocol 7.24

lseek(2) adds these:

       SEEK_DATA
              Adjust the file offset to the next location in the file greater than or equal to offset  containing  data.
              If offset points to data, then the file offset is set to offset.

       SEEK_HOLE
              Adjust  the  file  offset  to the next hole in the file greater than or equal to offset.  If offset points
              into the middle of a hole, then the file offset is set to offset.  If there is no hole past  offset,  then
              the  file  offset  is  adjusted  to the end of the file (i.e., there is an implicit hole at the end of any
              file).