LibCore: Add Core::System::drop_privileges()

In a few places we intentionally drop privileges to reduce the potential
security surface area of networked program, with the pattern of:

```
if (setgid(getgid()) || setuid(getuid()) {
    return 1;
}
```

We can make this a bit nicer to use by creating a wrapper.

Userland/Libraries/LibCore/System.h

@@ -109,6 +109,7 @@ ErrorOr<void> setgid(gid_t);
 ErrorOr<void> setegid(gid_t);
 ErrorOr<void> setpgid(pid_t pid, pid_t pgid);
 ErrorOr<pid_t> setsid();
+ErrorOr<void> drop_privileges();
 ErrorOr<bool> isatty(int fd);
 ErrorOr<void> symlink(StringView target, StringView link_path);
 ErrorOr<void> mkdir(StringView path, mode_t);