New command line option: --start-time=TIME

Expects a Unix time when the transmission should start. The given TIME
string is parsed, but not yet used.
Client: Store clock to use for packet timings in a variable

To apply start times given in Unix time, CLOCK_REALTIME must be used for
packet timings. However, CLOCK_MONOTONIC might be slightly more precise,
so the clock should be configurable, which requires a variable.
Remove unnecessary variable start_time_str

The parameter for --start-time is parsed directly from optarg, so
there's no need to store it in a separate variable.
Pass start time and clock to the client

The client does not use the start time yet, though.
Client: Use start time as passed from the caller

If both elements of start_time are zero, the current time is used
instead. This requires setting the default clock to CLOCK_REALTIME for
now, but it should be configurable later.
Add command line option --clock=CLOCK to set the clock to use

Valid parameters are "realtime" (CLOCK_REALTIME) and "monotonic"
(CLOCK_MONOTONIC). By default CLOCK_MONOTONIC is used, but if
--start-time is used, the clock is set to CLOCK_REALTIME.
Check for invalid combinations of --start-time and --clock=monotonic

A fixed start time in Unix time will not work with the monotonic clock,
because the latter does not necessarily follow Unix time. Abort
execution with an error message if the user tries to use the monotonic
clock in combination with --start-time.
List long command line options in the LUNA manual page
Merge branch 'starttime'

New client feature: Start the transmission at a fixed time, given in
Unix time
Documentation for --start-time and --clock command line options