<oo-admin-ctl-cgroups> Fix return value handling, code fixes/refactoring

This commit updates the script functions to properly capture return
codes from subcommands. The code has been refactored to accumulate
return codes in function-local variables and pass the accumulated
values via returns, instead of using global vars.

startall has been modified to properly react to the return code from
add_cgroup failure

Added optional parameter to usage string for status command