mkfile -- create large files efficiently

[Matej Cepl]

Hi,

I wanted to write this utility which exists on other Unices for
some time, but with introduction of posix_fallocate this actually
seems make sense now.

Eagerly waiting for comments and critcism -- this is my first
serious program in C which I make public, so I expect a lot of
both ;-).

Best,

Matěj Cepl