Flockit: add file locking to programs that don't have it.

This library exists solely because rsync doesn't have file locking.

It's not used like a normal library; you don't link against it, and you don't have to patch your source code to use it. It's inserted between your program and its libraries by use of LD_PRELOAD.

Example (with libflockit.so in the current directory)

$ env LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH LD_PRELOAD=libflockit.so FLOCKIT_FILE_PREFIX=test rsync SRC DEST