Switch from subprocess.Popen to os.system for OFED fork safety.
Introduce mechanism for switching method of calling external programs. Polish os.system implementation.
Rename env var INSTANT_CALL_METHOD to INSTANT_SYSTEM_CALL_METHOD and rename its values.
Fix remnants of INSTANT_SYSTEM_CALL_METHOD.
Add test for change of instant system calls
Merge branch 'blechta/ofed-fork'