gettext.c: work around us not using setlocale(LC_CTYPE, "")

gettext.c: work around us not using setlocale(LC_CTYPE, "")

Change the gettext setup code to arrange for messages to be emitted in
the user's requested encoding, but avoid setting LC_CTYPE from the
environment for the whole program.

In 107880a I removed our use of setlocale(LC_CTYPE, "") because of a
bug in vsnprintf in the GNU C Library [1].

Even if it wasn't for that bug we wouldn't want to use LC_CTYPE at
this point, because it'd require auditing all the code that uses C
functions whose semantics are modified by LC_CTYPE.

But only setting LC_MESSAGES as we do creates a problem, since we
declare the encoding of our PO files[2] the gettext implementation
will try to recode it to the user's locale, but without LC_CTYPE it'll
emit something like this on 'git init'

    Bj? til t?ma Git lind ? /hl/agh/.git/

Gettext knows about the encoding of our PO file, but we haven't told
it about the user's encoding, so all the non-US-ASCII characters get
encoded to question marks.

But we're in luck! We can set LC_CTYPE from the environment only while
we call nl_langinfo and bind_textdomain_codeset. That suffices to tell
gettext what encoding it should emit in, so it'll now say:

    Bjó til tóma Git lind í /hl/agh/.git/

And the equivalent ISO-8859-1 string will be emitted under a
ISO-8859-1 locale.

With this change way we get the advantages of setting LC_CTYPE (talk
to the user in his language/encoding), without the major drawbacks
(changed semantics for C functions we rely on).

However foreign functions using other message catalogs that aren't
using our neat trick will still have a problem, e.g. if we have to
call perror(3):

    #include <stdio.h>
    #include <locale.h>
    #include <errno.h>

    int main(void)
    {
        setlocale(LC_MESSAGES, "");
        setlocale(LC_CTYPE, "C");
        errno = ENODEV;
        perror("test");
        return 0;
    }

Running that will give you a message with question marks:

    $ LANGUAGE= LANG=de_DE.utf8 ./test
    test: Kein passendes Ger?t gefunden

In the long term we should probably see about getting that vsnprintf
bug in glibc fixed, and audit our code so it won't fall apart under a
non-C locale.

Then we could simply set LC_CTYPE from the environment, which would
make things like the external perror(3) messages work.

1. http://sourceware.org/bugzilla/show_bug.cgi?id=6530
2. E.g. "Content-Type: text/plain; charset=UTF-8\n" in po/is.po

Suggested-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>