Fix SEEK_END seeking twice as far as requested. Centralize lseek handling in
one place and avoid overflow bugs. Inode lseek handlers now only need to
handle SEEK_END with offset 0. Prevent the file offset from ever going below
zero or overflowing.

Character devices are now not seekable, but lseek will pretend they are, yet
always stay at the file offset 0. pread/pwrite on character devices will now
ignore the file offset and call read/write.

This change prevents character devices from being memory mapped, notably
/dev/zero can no longer be memory mapped. None of the current ports seem
to rely on this behavior and will work with just MAP_ANONYMOUS.

Refactor read and write system calls to have a shared return statement for
both seekable and non-seekable IO.

Fix file offset overflow bugs in read and write system calls.

Fix system calls returning EPERM instead of properly returning EBADF when
the file has not been opened in the ...