Implement wall clock time (rumprun-xen issue #13)

This change adds minios_clock_wall() as a global symbol to return wall
clock time from Xen. I did not reinstate the minios "gettime()" which is
available upstream as that relies on a definitition of struct timeval.

minios_monotonic_clock() is renamed to minios_clock_monotonic() for
consistency.

rumpuser_clock_gettime() is updated to support RUMPUSER_CLOCK_REWALL,
returning wall clock time.

All rumphyper calls to the minios NOW() macro are replaced with calls to
the properly namespaces minios_clock_monotonic().

Signed-off-by: Martin Lucina <martin@lucina.net>
Merge pull request #16 from rumpkernel/wip-timekeeping

Implement wall clock time (rumprun-xen issue #13)