cgtop: fix confusion in --help

cgtop: fix confusion in --help

When we say '-n --iterations=N' in --help, this means that the program
can be invoked with '-n N' or '--iterations=N' or '--iterations N'.
(The short option is specified without the argument.)

Here we tried to use '-p --order=path' to mean that the program can be
invoked with '-p' or '--order=path', but that is incompatible with the
established convention.

Also, indicate that the arg to --cpu is optional.

(cherry picked from commit cb406988ac587a0f2b75948f3fab860005d6bc76)
(cherry picked from commit 8523627f2197ef6d448d97832b946e42cd414fc3)
(cherry picked from commit 7f08d7160bbef89aac3df7f7fe93689965aaec6b)