Add support for AcceptEnv/SendEnv

with ssh the `LC_*` variables are inherited from the client session. with ssh3 they are system defaults.

```
$ grep -ri Env /etc/ssh/ssh*_config
/etc/ssh/ssh_config: SendEnv LANG LC_*
/etc/ssh/sshd_config:# Allow client to pass locale environment variables
/etc/ssh/sshd_config:AcceptEnv LANG LC_*
```

ssh3
```
glen@niblus:/etc/pam.d $ locale
LANG=
LANGUAGE=
LC_CTYPE="POSIX"
LC_NUMERIC="POSIX"
LC_TIME="POSIX"
LC_COLLATE="POSIX"
LC_MONETARY="POSIX"
LC_MESSAGES="POSIX"
LC_PAPER="POSIX"
LC_NAME="POSIX"
LC_ADDRESS="POSIX"
LC_TELEPHONE="POSIX"
LC_MEASUREMENT="POSIX"
LC_IDENTIFICATION="POSIX"
LC_ALL=
```

ssh

```
glen@niblus:~ $ locale
LANG=en_GB.UTF-8
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=
```

No source files or tests are named, so start by tracing the SSH3 client and server environment-handling entry points and comparing them with the documented SendEnv/AcceptEnv behavior. Reproduce the locale example, then add coverage showing that configured LANG and LC_* values are inherited rather than replaced by system defaults.

Written by the indexing model from the issue text.