This function allows the introduction of new severity classes which can be
addressed by the severity parameter of the fmtmsg function.
The severity parameter of addseverity must match the value
for the parameter with the same name of fmtmsg, and string
is the string printed in the actual messages instead of the numeric
value.
If string is NULL the severity class with the numeric value
according to severity is removed.
It is not possible to overwrite or remove one of the default severity
classes. All calls to addseverity with severity set to one
of the values for the default classes will fail.
The return value is MM_OK if the task was successfully performed.
If the return value is MM_NOTOK something went wrong. This could
mean that no more memory is available or a class is not available when
it has to be removed.
This function is not specified in the X/Open Portability Guide although
the fmtsmg function is. It is available on System V systems.
