attr: change validity check for attribute names to use positive logic

attr: change validity check for attribute names to use positive logic

Convert 'invalid_attr_name()' to 'attr_name_valid()' and use positive
logic for the return value.  In addition create a helper function that
prints out an error message when an invalid attribute name is used.

We could later update the message to exactly spell out what the
rules for a good attribute name are, etc.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>