Improve dmtcp integration for Debian.
Improve dmtcp integration for Debian.

* Only check for presence of dmtcp tools, not libs due to location
  difference. Moreover, Debian's dmtcp tools are not linked against
  those libs
* Set default snapshoting interval to zero. The previous half-hour
  interval may be completely inapproriate depending on the particular
  job and pool configuration (as any other value could be). Zero, will
  reduce the performance impact of the default configuration and still
  allow vanilla jobs to survive preemption.

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