- Make -M mask and -M time exclusive.

- Make -M mask and -M time exclusive.
- Correct some problems with packet construction.

  +--------+------------+----------+-------------+---------+
  |        |            |          |             |         |
  | IP Hdr | MINICMPLEN | phdr_len | TIMEVAL_LEN | payload |
  |        |            |          |             |         |
  +--------+------------+----------+-------------+---------+
  |        |                       |                       |
  |<- IP ->|<------- ICMP -------->|<------ datalen ------>|

My previous changes tried to mess around with 'datalen' instead of
modifying 'phdr_len'.

I'm including this nice ASCII diagram (from Maxim) to further clarify things
in CVS history.

Submitted by:	 Maxim Konovalov <maxim@macomnet.ru>