Set umask only if explicitly specified

[mike.dld]

Although #2234 was about daemon only, ticket component got changed to libtransmission and changes made affected all the clients. However, GUI clients should most probably inherit their umask. Attached patch makes "umask" parameter in settings.json optional, and if not present umask won't be modified and parameter won't be added to configuration file on save. The change won't affect existing installations, where "umask" is already present in settings.json, but for new installations users would need to add the parameter explicitly if they want to change it from default (inherited) one.

[mike.dld]

On a second thought, I decided to always save "umask" parameter to settings json and always return it on settings request. I guess Shift web interface (and it may not be the only one) shows all the available parameters, and "umask" won't be there for modification if I skip it. But parameter value of -1 would still mean that umask shouldn't be changed. Patch updated.