man: netsniff-ng: Minor fixes

Fix wording in some places, make some sentences a bit shorter and
replace "on default" by "by default".

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
man: trafgen: Complete documentation for option -p/--cpp

Add complete description of the commandline option -p/--cpp to the
manpage.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
man: trafgen: Minor fixes

Replace "on default" by "by default", make it a bit more clear what the
seed in the -E/--seed option is for and mention exit after display of
information on --version and --help.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>