core/execute: report invalid environment variables from files

Because "export key=val" is not supported by systemd, an error is logged
where the invalid assignment is coming from.

Introduce strv_env_clean_log() to log invalid environment assignments,
where logging is possible and allowed.

parse_env_file_internal() is modified to allow WHITESPACE in keys, to
report the issues later on.

src/core/execute.c

@@ -1770,6 +1770,8 @@ int exec_context_load_environment(const ExecContext *c, char ***l) {
                                 strv_free(r);
                                 return k;
                          }
+                        /* Log invalid environment variables with filename */
+                        p = strv_env_clean_log(p, pglob.gl_pathv[n]);
 
                         if (r == NULL)
                                 r = p;