Simple single-file FUSE implementation of copy-on-write and optionally copy-on-read using sparse files as storage to copyup

Fusecoraw added to support copy-on-read functionality. Credits go to original authors as their code is re-used in order to achieve copy-on-read.

Fusecow example session:
# mount | grep "on / "
/dev/sda8 on / type reiserfs (rw,user_xattr,usrquota,acl)
# touch mountpoint_file
# ./fusecow /dev/sda8 mountpoint_file write_file
# reiserfsck mountpoint_file
Replaying journal: Trans replayed: mountid 636, transid 14679519, desc 901, len 2, commit 904, next trans offset 887
Trans replayed: mountid 636, transid 14679520, desc 905, len 1, commit 907, next trans offset 890
...

# ls -lh
-rw------- 1 root disk  30G Jan 22 17:01 mountpoint_file
-rwxr-xr-x 1 root root  29G Jan 23 06:01 write_file
-rwxr-xr-x 1 root root 472K Jan 23 06:01 write_file.map

# du -sh *
32M	mountpoint_file
31M	write_file
281K	write_file.map

# mkdir mnt && mount -o loop mountpoint_file mnt
# dd if=/dev/urandom of=mnt/usr/local/etc/qqqqq bs=1M count=32

# umount mnt
# fusermount -u mountpoint_file
# du -sh *
0	mountpoint_file
65M	write_file
289K	write_file.map

# ls /usr/local/etc/qqqqq
ls: cannot access /usr/local/etc/qqqqq: No such file or directory

# ./fusecow /dev/sda8 mountpoint_file write_file
# mount -o loop mountpoint_file mnt
# ls -lh mnt/usr/local/etc/qqqqq
-rw-r--r-- 1 root root 32M Jan 23 06:05 mnt/usr/local/etc/qqqqq



Fusecoraw example session:

# touch mountpoint_file
# /fusecoraw /dev/sda8 mountpoint_file write_file read_file
# mount mountpoint_file /mnt/mountpoint
# ls /mnt/mountpoint
files

# umount /mnt/mountpoint
# umount mountpoint_file
# mount /dev/null mountpoint_file write_file read_file
# mount mountpoint_file /mnt/mountpoint
# ls /mnt/mountpoint
files

Limitations:
1. Slow
2. Cannot grow files yet
3. Unstable