#8 recvfile() should watch for "sparse" input

[Mikhail T.]

When receiving files, recvfile should check, if an block-size of the input consists of zeros. If such a block is detected, it should not be written out, but simpled seeked over (or, if using mmap -- ignored).

This would allow the filesystem to store the sparse files, potentially saving quite a bit of space. This behavior should either be always on, or triggerable by an option to recvfile().