mount.nfs: Do not segfault because of kernel version

mount.nfs segfaults if kernel version number does not contain
at least 3 components delimited with a dot.

Avoid this by matching up to three unsigned integers inialised
to zero, separated by dots.

A version that does not start with an integer is probably a future
version where the versioning evolved to another scheme.
Return UINT_MAX which is guaranteed to be higher than existing
versions. This would also make it possible to easily identify
versions that do not start with an integer.

Signed-off-by: Luk Claes <luk@debian.org>
Signed-off-by: Steve Dickson <steved@redhat.com>