minijail0: fix multiple data options with the -k mount

The recent fix for optional fields (fea05c6dcac6b69e9cc7ba03ee9c3d074)
broke passing in multiple mount options with the -k flag.  Fix it and
add a unittest to prevent regression.

Bug: None
Test: `make check` passes
Test: `./minijail0 -v -k none,/tmp,tmpfs,0xe,uid=10,mode=0755 /bin/ls -al /tmp/` works again
Change-Id: Ia9fe9dec4a16654fc9a32414586a8c39d22692eb
3 files changed