core: find a host UTF-8 locale

Some packages really want to use an UTF-8 locale, or they break.

However, there is no guarantee that any given locale is available on a
system. For example,, while most mainstream distros (Debian and
derivatives, Fedora...) do have the generic, language-agnostic C.UTF-8
locale, Gentoo does not provide it.

So, find the first UTF-8 locale available on the system, and take any
that is available. We however do favour using the user-set current
locale, then using the language-agnostic C.UTF-8, and eventually any
random UTF-8 locale.

Note: we only need to enforce LC_ALL, because setting it implies
everything else:
    http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_02



    """
    1. If the LC_ALL environment variable is defined and is not null,
    the value of LC_ALL shall be used.
    """

[Peter: use same regexp as in dependencies.sh]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>