It is now possible to enforce the suffix of a file.
Additionally, there is a check so that files named "/dev/null"
are not added any suffix nor backed up.

These changes are useful to implement single-file multiple walkers

Mode is redundant since:
* read or write can be decided depending if file is IFile or OFile
* append/non-append is decided based on RESTART option

Files with gz extension are transparently compressed and decompressed.

Closes #17

I set it to 2 instead of 2.0, so that it will not be necessary
to update it

clang -Wextra-semi