build: Pacify GCC 5 on 32-bit hosts

build: Pacify GCC 5 on 32-bit hosts

This pacifies GCC 5 in a better way, without disabling diagnostics.
* src/df.c (main): Tell compiler that optind is positive.
* src/shred.c (known): New function.
(dopass): Go back to off_t for file sizes.
Avoid integer overflow if we run off the end of the file.
Tell compiler that a write cannot write more bytes than requested.