ssh kitten: Allow to configure HOME environment variable

Useful if the user does not have a home directory or the directory is
not writable.

shell-integration/ssh/bootstrap.py

@@ -20,8 +20,13 @@ echo_on = int('ECHO_ON')
 data_dir = shell_integration_dir = ''
 request_data = int('REQUEST_DATA')
 leading_data = b''
-HOME = os.path.expanduser('~')
 login_shell = pwd.getpwuid(os.geteuid()).pw_shell or os.environ.get('SHELL') or 'sh'
+export_home_cmd = b'EXPORT_HOME_CMD'
+if export_home_cmd:
+    HOME = base64.standard_b64decode(export_home_cmd).decode('utf-8')
+    os.chdir(HOME)
+else:
+    HOME = os.path.expanduser('~')
 
 
 def set_echo(fd, on=False):