CRS/CRIU: add code to actually checkpoint a process

This adds the code to actually checkpoint a process using CRIU
with the necessary variables to control the behaviour.

Right now only --np 1 is supported and --mca oob tcp.

Following parameters are supported:

* crs_criu_log: name of the log file
* crs_criu_log_level: verbosity level in the log file
* crs_criu_tcp_established: C/R established TCP connections
* crs_criu_shell_job: C/R shell jobs
* crs_criu_ext_unix_sk: allow external unix connections
* crs_criu_leave_running: leave tasks in running state after checkpoint