track contents of growing log files

[Zugschlus]

This is https://sourceforge.net/p/aide/feature-requests/23/

for growing log files (option "S"), aide should be able to build checksums for the file's contents. On the next aide run, aide knows from the database the size the file had when the checksume found in the database where built, so it can verify whether the file contents has changed in the part that was already present when the reference database was built.