fix bad length counter, fix handling null bytes, refactor

[amfleurke]

There were two problems for me that I fixed in this version:

• counting the length of the session string was done before escaping, so it did not count all bytes.
• null bytes were not properly inserted.
• skip non-session files (in case you have your session files mixed with other files in a dir

I also refactored a bit:

• use function instead of recursive call. Is better readable
• echo what the script does, don't echo 'find: no access to file xxx' if you have (other) files with wrong permission in the session files dir