Added -messageid switch to send(1) and post(8). This allows selection

Added -messageid switch to send(1) and post(8).  This allows selection
of the style to use for generated Message-ID and Content-ID header
fields.  The default localname style is pid.time@localname, where time
is in seconds, and matches previous behavior.  The random style
replaces the localname with some (pseudo)random bytes and uses
microsecond-resolution time.