some coverity fixes plus fix fdx help (#328)

* coverity fixes

cmdhflegic- indications are the i in calls to data_buf[i] could = 1052
and overflow the array.
cmdhfmfhard - +1 to add space for string null terminator - should we add
the 0 terminator value too?
reveng.c - memory leak
util.c - fix potential overflow of array buf[]
util_posix.c - possible integer overflow

* fix help errors

* fix sprint_hex_ascii

again
and this function is not even used anywhere... yet...