Support readv and writev system calls

[rick-masters]

Please support readv and writev system calls.
These are used by musl internally to implement common libc I/O routines.

I am attaching a program for testing on Fiwix.

You can test it like so:

cc test-readv.c -o test-readv
rm -f testout.txt
./test-readv

The correct output is:

fd: 3
wrote 12
read return: 12
read str: /hello world!/

test-readv.c.gz

[mikaku]

Thank you very much.