locale: fix environment checks: LC_ALL has precedence over LC_CTYPE

'man 7 locale' describes following precedence:

If the second argument to setlocale(3) is an empty string, "", for
the default locale, it is determined using the following steps:

1. If there is a non-null environment variable LC_ALL, the value of
LC_ALL is used.

2. If an environment variable with the same name as one of the
categories above exists and is non-null, its value is used for that
category.

3. If there is a non-null environment variable LANG, the value of
LANG is used.

So, if LC_ALL is set, it is used regardless of LC_CTYPE or LANG.
Hence, when suggesting users what to change when their locale is invalid, we
should also first check LC_ALL instead of LC_CTYPE.