filesystem: switch to using /proc/self/mountinfo
filesystem: switch to using /proc/self/mountinfo

Change loadMountInfo() to load the mounts directly from
/proc/self/mountinfo, rather than use the mntent.h C library calls.

This is needed for correct handling of bind mounts and of "/dev/root",
since /proc/self/mountinfo has extra fields which show the mounted
subtree and the filesystem's device number.  /proc/mounts lacks these
fields, and the C library calls can't provide them.

To start, this patch just switches to using /proc/self/mountinfo,
without doing anything with the extra fields yet.

As a bonus, this eliminates all C code in mountpoint.go.

google/fscrypt,google/fscrypt,google/fscrypt