BSD mountinfo implementation is unsound

There are two problems:

• According to the documentation of reflect.SliceHeader it's never safe to modify or otherwise make use of the contents of SliceHeader:

  SliceHeader is the runtime representation of a slice. It cannot be used safely or portably and its representation may change in a later release. Moreover, the Data field is not sufficient to guarantee the data it references will not be garbage collected, so programs must keep a separate, correctly typed pointer to the underlying data.

  I'm sure that this is more of a CYA statement than anything else, but it does mean that technically our usage of this is unsound -- and ultimately the fix is just to switch to a C-style loop over the pointers.

• getmntinfo modifies a global variable, which means that if multiple goroutines try to get mountinfo at the same time we will end up potentially modifying the global structure during iteration. We could work around this by mutexing it or something, but a simpler solution would be to just use getfsstat(2) which allows us to pass our own allocated array.

Start by locating the BSD mountinfo implementation and its use of getmntinfo and reflect.SliceHeader. Review the SliceHeader documentation and the getfsstat(2) interface, then verify the result with the existing tests or mountinfo checks. Done means avoiding SliceHeader access and preventing concurrent iteration over a shared mountinfo structure.