cmfs-tools: implement cmfs_file_read() in libcmfs

This commit implements cmfs_file_read() in libcmfs/fileio.c, which read
a file region (may be whole file) from disk into a memory buffer.
cmfs-tools: implement more routines into libcmfs/cached_inode.c

This commit implement 2 stub routines in libcmfs/cached_inode.c:
- cmfs_read_cached_inode()
- cmfs_free_cached_inode()
All code is copied and modified from ocfs2-tools/libocfs2/cached_inode.c
cmfs-tools: declare cmfs_extent_map_get_blocks() in cmfs.h

This commit adds 2 function declarations into include/cmfs/cmfs.h,
- cmfs_bitmap_free()
- cmfs_extent_map_get_blocks()
cmfs-tools: update libcmfs/Makefile.am

Since new files (bitmap.c extent_map.c) are added into libcmfs,
Makefile.am should be updated to include these 2 files into build tree.