trafgen: set interpacket gap in time units (s/ms/us/ns)

Add ability to set IGP in time units in seconds, milliseconds,
microseconds, and nanoseconds by appending a postfix to --gap e.g. --gap
100ms. Also, update the man page and trafgen.zsh to reflect the
changes.

[Fix whitespaces, coding style and minor wording changes -- tklauser]

Signed-off-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>