Allow for rlimit cpu to specified which is now supported by the kernel.

Previously the rlimit units where limited to K, M, G and would fail when
KB, MB, GB where used.  Allow for both, also allow for units on lengths
of time, by specifying "seconds", "minutes", "hours".. or any unique subset
eg. "s", "sec", "m", "min", "h", "hour" ..

NOTE:
This patch does not extend rlimits to be able to handle setting of tasks
that are confined by other profiles.

Signed-off-by: John Johansen <john.johansen@canonical.com>