fix UB resulting from 32/64bit type mismatch in format strings
Makefile: all .o files need to be rebuilt when hexedit.h changes
remove misleading INT macro - it expanded to off_t
fix fd leak
mark.c: remove superfluous strlen call
hexStr... use size_t where size_t is needed
mark.c: check malloc return value
searchA: use size_t
fix prototypes of mymem*
make mymem* macros if available in libc
update README
remove superfluous libgen.h inclusion in header

the 2 TUs using it include it themselves already.
remove line-end witespace noise