Added option --check-inhibitors for non-tty usage

As described in #2680, systemctl did ignore inhibitors if it is not
attached to a tty to allow scripts to ignore inhibitors automatically.
This pull request preserves this behavior but allows scripts to
explicit check inhibitors if required.

The new parameter '--check-inhibitors=yes' enables this feature.
The old parameter '-i'/'--ignore-inhibitors' was deprecated in favor
of '--check-inhibitors=no', the default behaviour can be specified
with '--check-inhibitors=auto'.
The new parameter is also described in the documentations and shell
completions found here.