latency-test: typo for $1 lt 1000 computation

According to help text:
"Times without a suffix and less than 1000 are taken to be in us"

Signed-off-by: Dewey Garrett <dgarrett@panix.com>
latency-test: include -h|--help option

also:
  parse_time(): consider inputs as lower case
                to support uS,mS, etc.

also: allows testing single thread using either "-" or 0 for other period
      as mentioned in commit 5829d4cb

Signed-off-by: Dewey Garrett <dgarrett@panix.com>