Use filesystem::path instead of manual string tinkering

Where possible, use boost::filesystem::path instead of std::string or
char* for filenames. This avoids a lot of manual string tinkering, in
favor of path::operator/.

GetDataDir is also reworked significantly, it now only keeps two cached
directory names (the network-specific data dir, and the root data dir),
which are decided through a parameter instead of pre-initialized global
variables.

Finally, remove the "upgrade from 0.1.5" case where a debug.log in the
current directory has to be removed.