Implement Mounted() optimization using openat2(RESOLVE_NO_XDEV) #25

Currently, Mounted() have to consult mountinfo to figure out if the argument is a mount point.

This mountinfo parsing can be avoided by doing two stat() calls and comparing dev_t. Unfortunately, it is not working for bind mounts.

Now, with openat2(RESOLVE_NO_XDEV), we might be able to implement a trick to do a similar check for bind mounts, thus eliminating the need to parse mountinfo entirely.