Change type of struct complaints::series

Change type of struct complaints::series

Found while processing the C++ enum changes.  It seems like series
should be of type enum complaint_series, instead of adding a cast.

Redundant and out of date comments are also removed.

gdb/ChangeLog:

	* complaints.c (enum complaint_series): Add newlines and remove
	out of date comment.
	(struct complaints) <series>: Change type to enum
	complaint_series and remove out of date comment.
	(symfile_complaint_hook): Use equivalent enum value
	ISOLATED_MESSAGE instead of 0.