MINOR: tools: add fgets_from_mem

MINOR: tools: add fgets_from_mem

Add fgets_from_mem() helper to read lines from configuration files, stored now
as memory chunks. In order to limit changes in the first-level parser code
(readcfgfile()), it is better to reimplement the standard fgets, i.e. to
have a fgets, which can read the serialized data line by line from some memory
area, instead of file stream, and can keep the same behaviour as libc
implementations fgets.