MFC r331616: vfs_donmount: in certain cases try r/o mount if r/w mount fails

If the operation is not an update, if neither r/w nor r/o mode is
explicitly requested, if the error code hints at the possibility of the
media being read-only, and if the fallback is allowed, then we can try
to automatically downgrade to the readonly mode.

This is especially useful for auto-mounting of removable media that
sometimes can happen to be write-protected.

The fallback to r/o is not enabled by default. It can be requested on a
per-mount basis with a new mount option, 'autoro'. Or it can be
globally allowed by setting vfs.default_autoro.

stable/10 note: this branch does not have SYSCTL_BOOL, so SYSCTL_INT is
used instead.

Relnotes: yes