Implemented a new ini plugin

Because the simpleini plugin did not work too well for mounting the specification file and even failed to parse other ini files (e.g. samba), I wrote a new one. It uses a slightly patched version of the very small inih library (directly embedded). Although it works fine for now, it is still work in progress though. 

Improvements compared to simpleini:
* graceful handling of sections (keys are subkeys of their sections)
* support for comments (comments, are appended to the next non-comment key)
Not exactly. It is a slight adjustment to the version used in the augeas plugin. I am trying to experiment with the different options, but each option seems to have a major shortcoming. Integrating the condition in the macro (as done here) allows for really concise error checking, as shown in line 181. However, additional resources cannot be freed in the embedded if statement. Therefore this option cannot be used in each situation either.