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.cpp

@@ -726,6 +726,17 @@ ErrorOr<pid_t> setsid()
     return rc;
 }
 
+ErrorOr<void> drop_privileges()
+{
+    auto gid_result = setgid(getgid());
+    auto uid_result = setuid(getuid());
+
+    if (gid_result.is_error() || uid_result.is_error())
+        return Error::from_string_literal("Failed to drop privileges");
+
+    return {};
+}
+
 ErrorOr<bool> isatty(int fd)
 {
     int rc = ::isatty(fd);