Go back to allowing ``.'' as a username.group separator for backward

Go back to allowing ``.'' as a username.group separator for backward
compatibility.  : is still the documented non-ambiguous approach.  The
algorithm used will correctly parse david.obrien.staff as strrchar() is
used, and in my mind more people would use a ``.'' in the username than
the group name.

Convinced by argument and patch by: sheldonh (with slight changes by me)