Safe quoting for concatenating constant strings into patterns.
General improvements for Lua 5.1 and 5.2 coexistance:
* Add --versioned-rocks-dir option to Unix installer, which makes it use paths such as /lib/luarocks/rocks-5.X, and /etc/luarocks/config-5.X.lua (where X is 1 or 2).
* Make configure script on Unix autodetect the presence of previous LuaRocks installations and adapt accordingly to avoid conflicts.
* Support luarocks.site_config_5_X for users who wish to run two versions from the same source tree (may be useful for LR development).
* Try to load config-5.X.lua from home directory before trying config.lua.