Add shortcuts for setting time stamp precision

Add shortcuts for setting time stamp precision

Add two shortcuts (as long-options) for setting the time stamp
precision: --micro and --nano.  While adding these options, reshuffle
the usage message to group up the options related to time stamp, and
removing the macro TIME_STAMP_PRECISION_USAGE.

If setting the time stamp precision is not supported by the available
libpcap, the usage won't mention these options (including
--time-stamp-precision), but they will still be presented in the
manpage.