LibCore: Make get_current_user_home_path() return String & close passwd

This API was returning a "const char*" and it was unclear who took care
of the underlying memory. Returning a String makes that obvious.

Also make sure we close the /etc/passwd file when we're done with it.

Applications/Launcher/main.cpp

@@ -24,7 +24,9 @@ void handle_sigchld(int)
 
 int main(int argc, char** argv)
 {
-    chdir(get_current_user_home_path());
+    if (chdir(get_current_user_home_path().characters()) < 0)
+        perror("chdir");
+
     GApplication app(argc, argv);
 
     signal(SIGCHLD, handle_sigchld);