core/execute: report invalid environment variables from files

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.