gant: write man page

replace README
gant: fix a build warning

src/gant/gant.c: In function ‘dump_data’:
src/gant/gant.c:268:7: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 3 has type ‘size_t’ [-Wformat]
gant: fix a bunch of build warnings

antpm/src/gant/antlib.cpp:233:24: error: assigning to 'unsigned char *' from incompatible type 'void *'
                                                 burstbuf[chan] = malloc(BSIZE);
                                                                ^ ~~~~~~~~~~~~~
antpm/src/gant/antlib.cpp:247:25: error: assigning to 'unsigned char *' from incompatible type 'void *'
                                                         burstbuf[chan] = malloc(BSIZE);
                                                                        ^ ~~~~~~~~~~~~~
antpm/src/gant/antlib.cpp:256:25: error: assigning to 'unsigned char *' from incompatible type 'void *'
                                                         burstbuf[chan] = realloc(burstbuf[chan], bused[chan]+BSIZE);
                                                                        ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

antpm/src/gant/antlib.c: In function ‘commfn’:
antpm/src/gant/antlib.c:366:20: warning: unused parameter ‘arg’ [-Wunused-parameter]
antpm/src/gant/antlib.c: In function ‘ANT_Initf’:
antpm/src/gant/antlib.c:406:33: warning: unused parameter ‘baud’ [-Wunused-parameter]
antpm/src/gant/antlib.c: In function ‘ANT_Init’:
antpm/src/gant/antlib.c:435:30: warning: unused parameter ‘baud’ [-Wunused-parameter]
antpm/src/gant/antlib.c: In function ‘ANT_AssignChannelEventFunction’:
antpm/src/gant/antlib.c:618:38: warning: unused parameter ‘chan’ [-Wunused-parameter]

antpm/src/gant/gant.c: In function ‘chevent’:
antpm/src/gant/gant.c:821:9: warning: variable ‘newdata’ set but not used [-Wunused-but-set-variable]

antpm/src/gant/antlib.c: In function ‘ANT_AssignChannelEventFunction’:
antpm/src/gant/antlib.c:619:38: warning: unused parameter ‘chan’ [-Wunused-parameter]
gant: more build warning fixes

e.g.:
antpm/src/gant/gant.c:35:21: warning: comma at end of enumerator list [-pedantic]
antpm/src/gant/gant.c: In function ‘dump_data’:
antpm/src/gant/gant.c:264:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
antpm/src/gant/gant.c:267:24: warning: pointer of type ‘void *’ used in arithmetic [-pedantic]
antpm/src/gant/gant.c:267:33: warning: pointer of type ‘void *’ used in arithmetic [-pedantic]
antpm/src/gant/gant.c:275:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
antpm/src/gant/gant.c:284:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]