libcsupport: Add and use rtems_libio_iovec_eval()
Filesystem: Add readv/writev handlers

The readv() and writev() support was implemented in terms of multiple
calls to the read and write handlers.  This imposes a problem on device
files which use an IO vector as single request entity.  For example a
low-level network device (e.g. BPF(4)) may use an IO vector to create
one frame from multiple protocol layers each with its own IO vector
entry.
Filesystem: Use default kqfilter and poll handler
libcsupport: Accept NULL for zero-length entries