There are two write-only magic locale C strings for `setlocale`
- `""`, which means "set the locale based on the environment"
- nullptr (represented in PHP as the string `"0"`), which means
  "return the current locale"

On a fresh request, `setlocale(LC_ALL, "0")` previously returned `""`,
but
- the initial locale is `"C"`, not inherited from the environment (this matches
  C and Python, but PHP inherits from the environment by default)
- `""` should never be *returned* as a locale

Additionally, setting to `""` did set the locale to be inherited from the
environment, but calling `setlocale(LC_ALL, "")` would return the empty string
(indistinguishable from the initial state) rather than the locale.

This diff does not change the default locale or how setting works, but it does
correct the *indicated* locale, which was previously incorrect in these cases.

Future work will probably include
- adding a read-only getlocale()
- adding a read-only environmentlocale() or similar to make it possible to fetch
  it without changing the state
- add newlocale to allow using a specific locale without creating it

Test Plan:

```
$ locale
LANG=
LANGUAGE=
LC_CTYPE="en_GB.UTF-8"
LC_NUMERIC="en_GB.UTF-8"
LC_TIME="en_GB.UTF-8"
LC_COLLATE="en_GB.UTF-8"
LC_MONETARY="en_GB.UTF-8"
LC_MESSAGES="en_GB.UTF-8"
LC_PAPER="en_GB.UTF-8"
LC_NAME="en_GB.UTF-8"
LC_ADDRESS="en_GB.UTF-8"
LC_TELEPHONE="en_GB.UTF-8"
LC_MEASUREMENT="en_GB.UTF-8"
LC_IDENTIFICATION="en_GB.UTF-8"
LC_ALL=en_GB.UTF-8
```

debugger

```
$ hphp/hhvm/hhvm -m debug
hphpd> =setlocale(LC_ALL, "0")
"C"
hphpd> =setlocale(LC_ALL, "")
"en_GB.UTF-8"
hphpd> =setlocale(LC_ALL, "0")
"en_GB.UTF-8"
hphpd> =setlocale(LC_CTYPE, "fr_FR")
"fr_FR"
hphpd> =setlocale(LC_ALL, "0")
"LC_CTYPE=fr_FR;LC_NUMERIC=en_GB.UTF-8;LC_TIME=en_GB.UTF-8;LC_COLLATE=en_GB.UTF-8;LC_MONETARY=en_GB.UTF-8;LC_MESSAGES=en_GB.UTF-8;LC_PAPER=en_GB.UTF-8;LC_NAME=en_GB.UTF-8;LC_ADDRESS=en_GB.UTF-8;LC_TELEPHONE=en_GB.UTF-8;LC_MEASUREMENT=en_GB.UTF-8;LC_IDENTIFICATION=en_GB.UTF-8"
hphpd> =setlocale(LC_CTYPE, "")
"en_GB.UTF-8"
hphpd> =setlocale(LC_ALL, "0")
"en_GB.UTF-8"
```

For multiple requests

```
$ cat test.hack
<<__EntryPoint>>
function main(): void {
  var_dump(setlocale(LC_ALL, "0"));
  var_dump(setlocale(LC_ALL, ""));
  var_dump(setlocale(LC_ALL, "fr_FR"));
  var_dump(setlocale(LC_ALL, "0"));
}
$ hphp/hhvm/hhvm -m server -p 8080
```

and in another terminal...

```
$ curl http://localhost:8080/test.hack
string(1) "C"
string(11) "en_GB.UTF-8"
string(5) "fr_FR"
string(5) "fr_FR"
$ siege -c 50 -r 100 http://localhost:8080/test.hack # 5000 total requests with concurrency 50
...
$ curl http://localhost:8080/test.hack # Unchanged results
string(1) "C"
string(11) "en_GB.UTF-8"
string(5) "fr_FR"
string(5) "fr_FR"
```