MINOR: cfgparse: load_cfg_in_mem: take in account file size

MINOR: cfgparse: load_cfg_in_mem: take in account file size

Let's take in account the given file size, when its reported via stat.

It's very convenient for large configuration files, as this allows to
perform only the one memory allocation call for precisely needeed file size.
This also allows to perform only the one call to fread().

We need to provide to fread() file_stat.st_size + 1 to be able to grab EOF.
Like this it sets feof(f)=1 flag and this allows to exit from the loop
immediately, just after fread call.

If /dev/stdin or /dev/null is provided as a file, we continue to read the
configuration chunk by chunk, stat doesn't report the size.