In node.js server, RESTful API for querying and updating user records

Implement a RESTful API for querying and updating user records, and access it using libcurl from Unix-CB C code.

- Added libcurl to the autoconf requirements. It will typically already be installed on any Linux or MacOS system.
- In cbcfg, support overriding any cbconfig setting using an environment variable. This way, cb_server can override settings it needs like `HOMEREC` and `API_URL`.
- Added `updateUser` function to the datastore plugin classes.
- Added a RESTful API HTTP server to cb_server.js. Only functionality is reading and writing user records, in the KEY=value format used by Unix-CB.
- In Unix-CB, rewrote `rdrec` and `wrrec` to support accessing the RESTful API via libcurl, as well as reading/writing files with stdio like they used to.