Add support for default stratum split

[rleed]

The intention of this PR is to add support for a default stratum split. If no tilde is present in the stratum username, one is automatically added at the end of the username it behaves as with a blank modname. If a blank modname exists in the configuration, it will then serve as the default stratum split.

[luke-jr]

Seems like it would be better to just pass modname_len as 0?

[luke-jr]

Also, this means the username mod stuff is always enabled... Probably should have a bool on datum_config that gets set if "" is being used, so we're not wasting CPU on setups that don't use the feature.

[rleed]

Also, this means the username mod stuff is always enabled...

Doesn't if (datum_config.stratum_username_mod) disable it if it's not configured?

[rleed]

Seems like it would be better to just pass modname_len as 0?

// CAUTION: modname MUST be part of username_s following a tilde made me wary. The datum_stratum_mod_username function indexes the tilde character without checking that it exists first, which I think can't be assumed when just passing modname_len as 0.

[rleed]

Also, this means the username mod stuff is always enabled...

Doesn't if (datum_config.stratum_username_mod) disable it if it's not configured?

Ok, now I got it. I made the changes to skip the new logic if there is no default mod in the configuration.

[rleed]

Seems like it would be better to just pass modname_len as 0?

// CAUTION: modname MUST be part of username_s following a tilde made me wary. The datum_stratum_mod_username function indexes the tilde character without checking that it exists first, which I think can't be assumed when just passing modname_len as 0.

I've now modified the existing code to remove the caution above and pass a len of 0.