New code from airween to read offset frequency from fldigi via xmlrpc

- Added new configure option: --enable-fldigi-xmlrpc
- Added fldigixmlrpc.[ch] files to read the carrier value of Fldigi
- Modified affected parts of code, when indicated/used the RIG freq:
   * show frequency at right middle part in main window
   * store the correct freq when user grabs the spot
   * restore the correct freq when user loads a station from the bandmap

- finally add missing #include and include-guard