Detect if a debugger is already running.

- For the sake of genericity / clarity, renamed `eiki_gdb()` to
  `eiki_attach_debugger()`, renamed `EIKI_GDB_IF_TTY` to
  `EIKI_SIGNAL_HANDLER_DEBUGGER`, and renamed `EIKI_GDB_PATH` to
  `EIKI_DEBUGGER_PATH`. (All of the old names will continue to work, but
  new code is advised to use the new names.)

- On Linux and BSD derivatives (including Mac OS X),
  `eiki_attach_debugger()` will first attempt to detect if a debugger is
  already attached and fail silently if it is.

- Fixed some minor errors in the README.