Fall back to `userfaultfd` syscall on permission errors

[vapourismo]

Add option to fall back to userfaultfd system call when opening /dev/userfaultfd fails due to missing access rights. The default is unchanged.

It appears plenty of systems have very restricted access rights on /dev/userfaultfd yet allow the userfaultfd() system call for userspace-only operation. This option gives users of the library a little more leeway to support those systems out of the box.