sbin/hammer: Make hammer_parsedevs() initialize NumVolumes

sbin/hammer: Make hammer_parsedevs() initialize NumVolumes

- hammer_parsedevs() should initialize NumVolumes as well as
  initializing volumes. This is as simple as incrementing the
  counter each time setup_volume() is called.

- newfs_hammer can tell and initialize NumVolumes from argc,
  however low-level hammer commands that use hammer_parsedevs()
  can't do this because a blkdevs arg contains volumes joined
  by ':'. These commands need a way to initialize NumVolumes.