cache: Introduce StringCache, switch to unordered_map, default to memory cache if fs cache dir not given

sandbox/sandbox-linux.cpp

@@ -49,7 +49,11 @@ bool SandboxLinux::enable(std::vector<std::string> fsPaths)
 	}
 	for(unsigned int i = 0; i < fsPaths.size(); i++)
 	{
-		exile_append_path_policies(policy, EXILE_FS_ALLOW_ALL_READ | EXILE_FS_ALLOW_ALL_WRITE, fsPaths[i].c_str());
+		std::string &path = fsPaths[i];
+		if(path.size() > 0)
+		{
+			exile_append_path_policies(policy, EXILE_FS_ALLOW_ALL_READ | EXILE_FS_ALLOW_ALL_WRITE, path.c_str());
+		}
 	}
 	policy->drop_caps = 1;
 	policy->not_dumpable = 1;