Make # of samples configurable, fix build on sunos

 - Make # of samples configurable. Default # samples is 100M, and
   wrk try to allocate 1.6GB memory block for statistics. This
   could be a problem on small systems/VMs without enough RAM.
   I made # of samples to be configurable via command line options
   (-S). Default # of samples is also changed to 10M, now wrk uses
   140MB for statistics which ls affordable for small-meory
   environments.

 - Fix build on sunos (SmartOS). SmartOS ioctl.h defines FIONREAD
   only when BSD_COMP is defined, so I added it in Makefile.