Use a monotonic clock instead of a realtime clock

Using a realtime clock is a bad idea: it is affected by any kind of time
change, which can happen when the administrator modifies the system time,
or more simply when a laptop suspends to RAM and then wakes up from sleep.

With the current approach of using a realtime clock:

- if the system time jumps forward (e.g. when resuming after a
  suspend-to-RAM), bmon would take 100% CPU and display random graph data
  extremely fast, until it "catches up" with the new time.

- if the system time jumps backwards, bmon would freeze until *time*
  "catches up" to the point it was before.  bmon then (incorrectly)
  displays a spike in the graph, because lots of packets have been
  sent/received since the last update.

Instead of using gettimeofday(), switch to clock_gettime() with
CLOCK_MONOTONIC on systems that support it.  OS X does not provide
clock_gettime(), so this commit also adds a Mach-specific implementation.

This change has been tested on Linux 4.1 with glibc and musl, and on
FreeBSD 10.0-RELEASE-p12.

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