Use new struct MemArgs for -m, -n options [skip CI]

cmd.c:
- added struct MemArgs to store --memory <M> --nkmers <N> values
  - added associated methods cmd_mem_args_set_memory(), cmd_mem_args_set_nkmers()
- cmd_get_longopt_str(), cmd_long_opts_to_short() take buffer length argument
  for safety, and call string_buffer.c:string_safe_ncpy().

scripts/make-circos.pl
- print colour key with circos generation