added new RmSettings member iwd and new RmFile member offset.

RmSettings.iwd stores the initial working directory
  (robust way of reconstructing full path of relative paths)
RmFile.offsete stores the byte offset of the first byte of the file
  (relative to the start of the device).
added rm_fullname to get full file path of a file
Added filemap.c to get byte offset of a file on its device
Currently only used to get offset of the first byte of a file, but
in future may be useful to get offset of the last byte of the file
that we just finished reading, so that we can make a smart decision
about what file to read next.
Merge branch 'fiemap' of https://github.com/SeeSpotRun/rmlint into develop