Just like fat the test isn't wired up yet until we have the infrastructure
for it.

Imported from prex.  Note that the test isn't wired up yet as we'll need
more infrastructure changes to run it.

this avoids a clash with the C++ std::list type.

This has no functional benefit, it just allows testing the async
block layer and can be reverted later.

Struct bio gains a condition variable and a mutex to allow waiting for
I/O completions.  Still no support for returning errors, but that can
be added fairly easily.

I've also added an osv/ prefix to make sure we don't clash with system headers.

We probably should either adopt this globally or drop it, but I fear we'll run into
problems with the latter approach.

Various glibc tests need it.

This is inspired by the FreeBSD code, but nothing but the header and function
names are left of the original version.

Doesn't actually support real asynchronous execution as we're missing the concept
of waitqueues in osv as of now, but that will be added soon.

We want these in line buffered mode, but to get that we need to query the
console driver if it is a tty, and we can only do that after the VFS has
been brought up.

The test code assumes /tests contains shared objects.

This will allow for easy addition of the vectored and positional I/O
syscalls, as well as making the structure of the I/O path more similar
to *BSD and Solaris.

until we support symlinks lxstat is identical to stat