Configuration library and tool

[aquette]

libnutconf and nutconf are available in branch libconf:
https://github.com/networkupstools/nut/tree/libconf

[aquette]

The list of current features (available through nutconf) can be seen in the help message:
https://github.com/networkupstools/nut/blob/libconf/tools/nutconf/nutconf.cpp

Remaining tasks list:

• Document libnutconf API
• Implement upssched.conf handler
• Implement other missing features (maxage, deadtime, ..., named Vs positional)
• QA Regression Testing suite integration

@vencik: can you please complete the above list, as per our mails?