Allocate the `struct statfs` buffer on the heap instead of the stack,
fixing a -Wframe-larger-than warning.

Fix a -Wsign-compare warning with the GCC 32-bit builds.

Add a couple more headers for getfsstat(), as given by its manual page
(although it builds just fine without them).

…ant buffer

For the statfs buffer used by detectAllDrives(), only allocate what is
needed, instead of an arbitrary value.

Calling getfsstat() a first time with a NULL pointer will return the
number of mounted file systems. We can then use that a allocate a
buffer at the right size for the next getfsstat() call.

It looks like we can then reasonably expect MNT_NOWAIT to be enough
for the second call that immediately follows...

Idea from criezy.