core/exec-invoke: split out maybe_inherit_stdout_from_stdin(), use exec_input_is_inhe...

core/exec-invoke: split out maybe_inherit_stdout_from_stdin(), use exec_input_is_inheritable()

Note that exec_input_is_inheritable() rightfully refuses EXEC_INPUT_FILE,
in which case std_output would have been reset in service_fix_stdio()
already.

While at it, use the generic fallback logic of first trying user manager
stdout when stdin is not writable.