Replace optparse with argparse

* Since version 2.7 optparse is deprecated and replaced by argparse. This patch replaces optparse in the given files.
* Also added a small note in nipap.man.rst relating to the settings file.
* In the nipap-passwd tool you can now list a specific user (with option -u).

By the way the command nipapd --version doens't seem to work. I don't know how to fix it though and the error was already present without the patch.
```
Traceback (most recent call last):
  File "./nipapd", line 49, in <module>
    print "nipapd version:", nipap.__version__
NameError: name 'nipap' is not defined
```