configd: T6640: enforce in_session returns False under configd

The CStore in_session check is a false positive outside of a config
session if a specific environment variable is set with an existing
referent in unionfs. To allow extensions when running under configd and
avoid confusion, enforce in_session returns False.

python/vyos/configsource.py

@@ -204,6 +204,8 @@ def in_session(self):
         Returns:
             True if called from a configuration session, False otherwise.
         """
+        if os.getenv('VYOS_CONFIGD', ''):
+            return False
         try:
             self._run(self._make_command('inSession', ''))
             return True

src/services/vyos-configd

@@ -267,6 +267,8 @@ if __name__ == '__main__':
     cfg_group = grp.getgrnam(CFG_GROUP)
     os.setgid(cfg_group.gr_gid)
 
+    os.environ['VYOS_CONFIGD'] = 't'
+
     def sig_handler(signum, frame):
         shutdown()