Change the report signal numbers

[Bojun-Seo]

I suggest to change the report signal numbers from SIGUSR1 and SIGUSR2 to some other numbers greater than 32, which is not usually reserved for system. There are two reasons.
First, SIGUSR1 and SIGUSR2 are too common so some other library or program could easily overwrite heaptrace registered signal handler.
Second, to support another sort key. The report could be sorted in slope or r square order, if we accept to add SLR(simple linear regression) analysis on report(See #34 for details about adding SLR). We need to add another signal to report in slope and r square order but there is no SIGUSR3 and SIGUSR4.