Fix server data dir (/var/run/rstudio) not writable. Support multiple rstudio-server versions

[ccoulombe]

By default, the option --set-data-dir points to /var/run/rstudio which is not writable other then by sudo. Set the option to a temporary directory.

Support multiple rstudio-server versions, from v1.2.1335 and up

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[ryanlovett]

Thanks for this @ccoulombe ! In a similar PR, #118, I suggested it'd be better to create the data dir outside of db_config() and then pass the directory in as a parameter. I think this is more logical since the data dir is used for things other than the db. For example, something like this.

I like expanding the rserver parameters when they're supported by the help text.

[ccoulombe]

@ryanlovett I'll update according to your comments in the PR.

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[ryanlovett]

Thanks again @ccoulombe ! Sorry for the delay -- I had missed that you updated the PR.