poll.2, select.2: fix erroneous description of "available for write".

POSIX says: "POLLOUT Normal data may be written without
blocking.".  This is "may" is misleading, see the POSIX
write page:

    Write requests to a pipe or FIFO shall be handled in the
    same way as a regular file with the following exceptions:
    ...
    If the O_NONBLOCK flag is clear, a write request may cause
    the thread to block, but on normal completion it shall
    return nbyte.
    ...
    When attempting to write to a file descriptor (other than a
    pipe or FIFO) that supports non-blocking writes and cannot
    accept the data immediately:

    If the O_NONBLOCK flag is clear, write() shall block the
    calling thread until the data can be accepted.

    If the O_NONBLOCK flag is set, write() shall not block the
    thread. If some data can be written without blocking the
    thread, write() shall write what it can and return the
    number of bytes written. Otherwise, it shall return -1 and
    set errno to [EAGAIN].

The net result is that write() of more than 1 byte on a
socket, pipe or FIFO which is "ready" may block: write()
(unlike read!) will attempt to write the entire buffer and
only return a short write under exceptional circumstances.

Indeed, this is the behaviour we see in Linux:

897626152d
https://plus.google.com/103188246877163594460/posts/BkTGTMHDFgZ

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/poll.2

@@ -167,7 +167,10 @@ There is urgent data to read (e.g., out-of-band data on TCP socket;
 pseudoterminal master in packet mode has seen state change in slave).
 .TP
 .B POLLOUT
-Writing now will not block.
+Writing is now possible, though a write larger that the available space
+in a socket or pipe will still block (unless
+.B O_NONBLOCK
+is set).
 .TP
 .BR POLLRDHUP " (since Linux 2.6.17)"
 Stream socket peer closed connection,

man2/select.2

@@ -86,9 +86,10 @@ allow a program to monitor multiple file descriptors,
 waiting until one or more of the file descriptors become "ready"
 for some class of I/O operation (e.g., input possible).
 A file descriptor is considered ready if it is possible to
-perform the corresponding I/O operation (e.g.,
-.BR read (2))
-without blocking.
+perform a corresponding I/O operation (e.g.,
+.BR read (2)
+without blocking, or a sufficiently small
+.BR write (2)).
 .PP
 The operation of
 .BR select ()
@@ -131,8 +132,8 @@ available for reading (more precisely, to see if a read will not
 block; in particular, a file descriptor is also ready on end-of-file),
 those in
 .I writefds
-will be watched to see if a write will not block, and
-those in
+will be watched to see if space is available for write (though a large
+write may still block), and those in
 .I exceptfds
 will be watched for exceptions.
 On exit, the sets are modified in place