Added initial file for APB message.
APB message: initialize.
API extension: navi_print_offset().
API extension: navi_status_str().
APB sentence: implementation of generator.
API extension: navi_offset_NULL designating null field for offsets, headings and bearings.
APB message: composition of sentence without using validity flags.
Generator: generation of APB sentence.
Generator: fixed compilation error.
Parser: extended navi_parse_offset() function.
APB message: parser.
Parser: APB message.
API extension: navi_init_position() with null value.
API extension: navi_init_offset().
DTM sentence: update source code using new API.
API update: navi_datum_str().
API update: navi_datumsubdiv_str().
API update: navi_fixsign_str().
API update: navi_gsamode_str().
Fixed typos.