A required argument is passed to the "--dvm" directive to specify the
location of the DVM controller (e.g., "--dvm pid:12345") or by passing
the string "search" to instead search for an existing controller.

Supported options include:

* "search": directs the tool to search for available DVM controllers
  it is authorized to use, connecting to the first such candidate it
  finds.

* "pid:<arg>": provides the PID of the target DVM controller. This can
  be given as either the PID itself (arg = int) or the path to a file
  that contains the PID (arg = "file:<path>")

* "file:<path>": provides the path to a PMIx rendezvous file that is
  output by PMIx servers — the file contains all the required
  information for completing the connection

* "uri:<arg>": specifies the URI of the DVM controller, or the name of
  the file (specified as "file:filename") that contains that info

* "ns:<arg>": specifies the namespace of the DVM controller

* "system": exclusively find and use the system-level DVM controller

* "system-first": look for a system-level DVM controller, fall back to
  searching for an available DVM controller the command is authorized
  to use if a system-level controller is not found

Examples:

   prterun --dvm file:dvm_uri.txt --np 4 ./a.out

   prterun --dvm pid:12345 --np 4 ./a.out

   prterun --dvm uri:file:dvm_uri.txt --np 4 ./a.out

   prterun --dvm ns:prte-node1-2095 --np 4 ./a.out

   prterun --dvm pid:file:prte_pid.txt --np 4 ./a.out

   prterun --dvm search --np 4 ./a.out
