Using auto-generated links is dangerous, as there are multiple
definitions for syscalls (at least one on each book part).

So, reference them by their explicit reference.

I used this small script to help writing this patch:

for i in $(git grep -l "c:func:"); do perl -ne 's/\:c\:func:\`(open|close|read|poll|write|select|mmap|munmap|ioctl)\(\)`/:ref:`$1() <func-$1>`/; print $_' < $i >a && mv a $i; done
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>