ssh: set a valid locale in the ssh-serve process

In the SSH client configuration, the setting 'SendEnv' could contain variables
like 'LANG', 'LC_ALL', and others. This causes these environment variables
(with their values at the client-side) to be set in the server. However, not
every locale setting valid in the client, is also valid on the server.

This could lead to the error:
'locale.Error: unsupported locale setting'
when 'from mercurial import archival, merge as hg_merge, patch, ui' is
called.

Fix this problem by providing an ini setting 'ssh_locale' that the user can
set correctly, and which will be used to set LC_ALL and LANGUAGE in the
'kallithea-cli ssh-serve' process.

If an environment variable LC_ALL is set, it takes precedence over all other
'LC_xxx' variables, as well as over LANG. So, setting LC_ALL ensures that no
user setting of 'LC_xxx' or 'LANG' could influence ssh-serve badly.

There is one environment variable that might overrule LC_ALL, specifically
for showing messages: 'LANGUAGE'. GNU gettext lets it take precedence over
LC_ALL [1]:
"GNU gettext gives preference to LANGUAGE over LC_ALL and LANG for the
purpose of message handling"

So, also set LANGUAGE to the same value as we set LC_ALL to.


The principle of setting a specific locale in the server process to fix this
error, was first proposed by Dominik Ruf.

[1] https://www.gnu.org/software/gettext/manual/html_node/The-LANGUAGE-variable.html#The-LANGUAGE-variable