Associate build data with manually-specified rc file

Currently, the build data is stored globally when using the default rcfile, and when using a manually-specified rcfile, it is stored in the same directory as the rcfile. This generally works.

But if you have two rcfiles that live in the same directory, they inappropriately share build data, because each one stomps on the build date file generated by the other one. You can manually specify persistent-data-file in your config file to fix this, but that may be forgotten, not known about, or unwanted.

This commit fixes that by appending the name of the manually-specified rcfile to the build data file when not using persistent-data-file in the config file. This automatically associates each manually-specified rcfile with its own private store of build data, even when it lives in the same directory as another rcfile.

There is no change when using the default global rcfile behavior.

Fixes #117 (closed)