st: cureregr update

[Allen Buxton <abbuxton@earthlink.net>]

Thanks to Kit Baum an update to the package 'CUREREGR: module to 
estimate parametric cure regression' is available in the SSC archive.

The primary update is to allow no varlist as a valid option -like streg- 
to result in a constant only model.

Thank you,
Allen
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