perf bench: Harmonize all the -l/--nr_loops options

We have three benchmarking subsystems that specify some sort of 'number
of loops' parameter - but all of them do it inconsistently:

 numa:              -l/--nr_loops
 sched messaging:   -l/--loops
 mem memset/memcpy: -i/--iterations

Harmonize them to -l/--nr_loops by picking the numa variant - which is
also the most likely one to have existing scripting which we don't want
to break.

Plus improve the parameter help texts to indicate the default value for
the nr_loops variable to keep users from guessing ...

Also propagate the naming to internal variables.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1445241870-24854-13-git-send-email-mingo@kernel.org


[ Let the harmonisation reach the perf-bench man page as well ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>