Change system File Creation Mode for multi-user access

Overriding the File Creation Mode set by umask on Linux allows FW to create
directories and files with the permissions needed for group write access in
/var/lib/fieldworks. Safely resets the umask after creating files.
Completion of fix for FWNX-1002.

Change-Id: I6da15cc9aefd18f3bfb83a74c42db70fff9949b5