Interruptibility of file operations is strongly desirable in case of
slow storage access, e.g. mounted network share.
This commit introduces possibility to limit data quantity transferred by
'file' protocol at once. By default, old behaviour is preserved and data
is still tried to be transferred without block size limitation.

Note that file I/O operation still may block (or even freeze) inside of
single read(2) or write(2) operation.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>