systemctl: warn when importing environment variables with control characters

I don't think it is useful to warn about about environemnt variables where the user
explicitly configured some value. If they went through the effort of escaping the cc
to include it in the setting (e.g. Environment="VAR=\efoo"), and we pass this through,
there isn't anything to warn about. This also applies to 'systemctl set-environment',
where the variable name and value are passed as arguments.

The only case where the warning *might* be useful is where the user might be
surprised by the value. This occurs when importing variables from the inherited
environment, i.e. in 'systemctl import-environment'. In not convinced that this is
useful, since the user better control their shell environment anyway.

$ systemctl import-environment
Calling import-environment without a list of variable names is deprecated.
Environment variable $LESS_TERMCAP_mb contains control characters, importing anyway.
Environment variable $LESS_TERMCAP_md contains control characters, importing anyway.
Environment variable $LESS_TERMCAP_me contains control characters, importing anyway.
Environment variable $LESS_TERMCAP_se contains control characters, importing anyway.
Environment variable $LESS_TERMCAP_so contains control characters, importing anyway.
Environment variable $LESS_TERMCAP_ue contains control characters, importing anyway.
Environment variable $LESS_TERMCAP_us contains control characters, importing anyway.
Environment variable $ZZZ contains control characters, importing anyway.

(cherry picked from commit e9155cd0)