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.
Move a little more code inside #if HAVE_LIBCURL
Merge pull request #16 from sourcedelica/ggrossman/restful_api

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