MINOR: cfgparse: limit file size loaded via /dev/stdin

MINOR: cfgparse: limit file size loaded via /dev/stdin

load_cfg_in_mem() can continuously reallocate memory in order to load an
extremely large input from /dev/stdin, until it fails with ENOMEM, which means
that process has consumed all available RAM. In case of containers and
virtualized environments it's not very good.

So, in order to prevent this, let's introduce MAX_CFG_SIZE as 10MB, which will
limit the size of input supplied via /dev/stdin.