This function allows the introduction of new severity classes which can be
addressed by the severity parameter of the fmtmsg(3) function.
By default that latter function only knows how to print messages for severity
0-4 (with strings (none), HALT, ERROR, WARNING, INFO). This call attaches the
given string s to the given value severity. If s is NULL,
the severity class with the numeric value severity is removed. It is
not possible to overwrite or remove one of the default severity classes. The
severity value must be nonnegative.