Three bugfixes and a feature

Three bugfixes and a feature
  - blank lines no longer give: Unknown request ""
  - ^C just cancels the current line instead of killing the program
  - \\ is now valid in quoted strings (along with \nnn and \")
  - If you define USE_READLINE and tell it where to find includes
    and libraries, it will build in support for gnu readline. This
    isn't enabled by default, but I'm going to rebuild them with it,
    and it will be easily configurable in the future autoconfiscated
    version of the source tree.