Introduce the new nonblock API.

Introduce the new nonblock API.

This patch is an attempt to create a better API for reads from
non-blocking file descriptors. It adds readv_nonblock() and implements
read_nonblock() as a simple wrapper for readv_nonblock(). Both new
functions check the given file descriptor for readability and read
as much as possible until an error occurs or the buffer is full.

Two additional parameters are introduced: An fd_set and a result
pointer for the number of bytes that have been read successfully. The
optional fd_set pointer is used to have the fixup code for unreliable
returns of FD_ISSET() at one place only.

Having an extra parameter for storing the number of bytes read allows
to treat EOF as an error condition but EAGAIN as a normal condition.
This will simplify callers (dccp_recv and upd_recv) a bit.