remove 100-levels config nesting limit
The actual reason for this commit isn't the limit – there isn't much
point in using that much nesting – its in shutting up gcc mostly:
apt/apt-pkg/contrib/configuration.cc: In function ‘bool ReadConfigFile(Configuration&, const string&, const bool&, const unsigned int&)’:
apt/apt-pkg/contrib/configuration.cc:686:20: warning: cannot optimize loop, the loop counter may overflow [-Wunsafe-loop-optimizations]
string Stack[100];
^
by replacing this with C++s handy std::stack container (adapter).
Also cleans some whitespace noise from the file in the process.